cmake: restore X11/Wayland support on non-Linux

[jbeich]

Regressed by #4954. Disable platform gatekeeping as X11/Wayland are also supported on BSDs. CC @brad0

$ vkcube-wayland --validate
WARNING : VALIDATION - Message Id Number: 2044605652 | Message Id Name: VUID_Undefined
        Validation Warning: [ VUID_Undefined ] Object 0: handle = 0x82876a000, type = VK_OBJECT_TYPE_INSTANCE; | MessageID = 0x79de34d4 | Instance Extension VK_KHR_wayland_surface is not supported by this layer.  Using this extension may adversely affect validation results and/or produce undefined behavior.

        Objects - 1
                Object[0] - VK_OBJECT_TYPE_INSTANCE, Handle 0x82876a000

Selected GPU 0: Intel(R) HD Graphics 530 (SKL GT2), type: IntegratedGpu
ERROR : VALIDATION - Message Id Number: -2145442690 | Message Id Name: VUID-vkGetPhysicalDeviceSurfaceSupportKHR-surface-parameter
        Validation Error: [ VUID-vkGetPhysicalDeviceSurfaceSupportKHR-surface-parameter ] Object 0: handle = 0x82876a000, type = VK_OBJECT_TYPE_INSTANCE; | MessageID = 0x801f247e | Invalid VkSurfaceKHR Object 0x828746c30. The Vulkan spec states: surface must be a valid VkSurfaceKHR handle (https://www.khronos.org/registry/vulkan/specs/1.3-extensions/html/vkspec.html#VUID-vkGetPhysicalDeviceSurfaceSupportKHR-surface-parameter)

        Objects - 1
                Object[0] - VK_OBJECT_TYPE_INSTANCE, Handle 0x82876a000

ERROR : VALIDATION - Message Id Number: 1567320034 | Message Id Name: UNASSIGNED-Threading-Info
        Validation Error: [ UNASSIGNED-Threading-Info ] Object 0: handle = 0x828746c30, type = VK_OBJECT_TYPE_SURFACE_KHR; | MessageID = 0x5d6b67e2 | Couldn't find VkSurfaceKHR Object 0x828746c30. This should not happen and may indicate a bug in the application.

        Objects - 1
                Object[0] - VK_OBJECT_TYPE_SURFACE_KHR, Handle 0x828746c30

Segmentation fault

[ci-tester-lunarg]

Author jbeich not on autobuild list. Waiting for curator authorization before starting CI build.

[CLAassistant]

All committers have signed the CLA.

[ci-tester-lunarg]

CI Vulkan-ValidationLayers build queued with queue ID 1638.

[juan-lunarg]

Please amend your commit message and force push so it passes CI:

Checking commit: "f4d438f73 cmake: restore X11/Wayland support on non-Linux after 33935a1619dc"

The following subject line exceeds 64 characters in length.
     'cmake: restore X11/Wayland support on non-Linux after 33935a1619dc'

The first word of the subject line after the ':' character must be capitalized.
     'cmake: restore X11/Wayland support on non-Linux after 33935a1619dc'

[ci-tester-lunarg]

CI Vulkan-ValidationLayers build # 10198 running.

[ci-tester-lunarg]

Author jbeich not on autobuild list. Waiting for curator authorization before starting CI build.

[ci-tester-lunarg]

CI Vulkan-ValidationLayers build queued with queue ID 1688.

[ci-tester-lunarg]

CI Vulkan-ValidationLayers build # 10200 running.

[ci-tester-lunarg]

CI Vulkan-ValidationLayers build # 10200 passed.

[brad0]

@jbeich Thanks.