cmake: pass libusb include directory as well

In file included from src/input_common/gcadapter/gc_adapter.cpp:8:
src/./input_common/gcadapter/gc_adapter.h:11:10: fatal error: 'libusb.h' file not found
 #include <libusb.h>
          ^~~~~~~~~~

2 files changed, 2 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index cd40e5cfe..eb51d09dc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -333,6 +333,7 @@ endif()
 find_package(LibUSB)
 if (NOT LIBUSB_FOUND)
     add_subdirectory(externals/libusb)
+    set(LIBUSB_INCLUDE_DIR "")
     set(LIBUSB_LIBRARIES usb)
 endif()
 
diff --git a/src/input_common/CMakeLists.txt b/src/input_common/CMakeLists.txt
index 3bd76dd23..1d7e19a66 100644
--- a/src/input_common/CMakeLists.txt
+++ b/src/input_common/CMakeLists.txt
@@ -30,6 +30,7 @@ if(SDL2_FOUND)
     target_compile_definitions(input_common PRIVATE HAVE_SDL2)
 endif()
 
+target_include_directories(input_common SYSTEM PUBLIC ${LIBUSB_INCLUDE_DIR})
 target_link_libraries(input_common PUBLIC ${LIBUSB_LIBRARIES})
 
 create_target_directory_groups(input_common)