Externals/hidapi: Convert to submodule

.gitmodules

@@ -72,3 +72,6 @@
 [submodule "Externals/enet/enet"]
 	path = Externals/enet/enet
 	url = https://github.com/lsalzman/enet.git
+[submodule "hidapi-src"]
+	path = Externals/hidapi/hidapi-src
+	url = https://github.com/libusb/hidapi

Externals/hidapi/CMakeLists.txt

@@ -1,20 +1,20 @@
 project(hidapi)
 
-add_library(hidapi STATIC hidapi/hidapi.h)
+add_library(hidapi STATIC hidapi-src/hidapi/hidapi.h)
 dolphin_disable_warnings_msvc(hidapi)
-target_include_directories(hidapi PUBLIC hidapi)
+target_include_directories(hidapi PUBLIC hidapi-src/hidapi)
 
 if(APPLE)
-  target_sources(hidapi PRIVATE mac/hid.c)
+  target_sources(hidapi PRIVATE hidapi-src/mac/hid.c)
 elseif(MSVC)
-  target_sources(hidapi PRIVATE windows/hid.c)
+  target_sources(hidapi PRIVATE hidapi-src/windows/hid.c)
 else()
   find_package(LIBUDEV)
   if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND LIBUDEV_FOUND)
-    target_sources(hidapi PRIVATE linux/hid.c)
+    target_sources(hidapi PRIVATE hidapi-src/linux/hid.c)
     target_link_libraries(hidapi PRIVATE udev)
   else()
-    target_sources(hidapi PRIVATE libusb/hid.c)
+    target_sources(hidapi PRIVATE hidapi-src/libusb/hid.c)
     target_link_libraries(hidapi PRIVATE ${LIBUSB_LIBRARIES})
   endif()
 endif()