Work around gcc-8 warning with GetProcAddress

On MinGW starting from gcc-8, casting FARPROC returned by GetProcAddress
produces the following warning:

 cast between incompatible function types from 'FARPROC' {aka 'long long
 int ()()'} to 'void ( (*)(struct VkInstance_T *, const char *))(void)'

This change works around it by casting through a void(*)(void) type,
which seems to be the idiomatic way to silence this.

Fixes zeux#19.

Author: zeux

volk.c

@@ -33,7 +33,8 @@ VkResult volkInitialize(void)
 	if (!module)
 		return VK_ERROR_INITIALIZATION_FAILED;
 
-	vkGetInstanceProcAddr = (PFN_vkGetInstanceProcAddr)GetProcAddress(module, "vkGetInstanceProcAddr");
+	// note: function pointer is cast through void function pointer to silence cast-function-type warning on gcc8
+	vkGetInstanceProcAddr = (PFN_vkGetInstanceProcAddr)(void(*)(void))GetProcAddress(module, "vkGetInstanceProcAddr");
 #elif defined(__APPLE__)
 	void* module = dlopen("libvulkan.dylib", RTLD_NOW | RTLD_LOCAL);
 	if (!module)