libobs: Use system header notation for pthread.h include

Using a relative path for the pthread.h header by w32-pthreads breaks
compilation of plugins which include threading.h from libobs (as
w32-pthreads will exist at a different location relative to the
libobs header).

As w32-pthreads (and its include directory) is added to the libobs
target by CMake, the pthread.h header will be found even when using
system header notation.

Fixes https://github.com/obsproject/obs-studio/issues/7155

libobs/CMakeLists.txt

@@ -313,7 +313,7 @@ if(OS_WINDOWS)
   target_link_libraries(libobs PRIVATE Avrt Dwmapi winmm)
 
   if(MSVC)
-    target_link_libraries(libobs PRIVATE OBS::w32-pthreads)
+    target_link_libraries(libobs PUBLIC OBS::w32-pthreads)
 
     target_compile_options(libobs PRIVATE "$<$<COMPILE_LANGUAGE:C>:/EHc->"
                                           "$<$<COMPILE_LANGUAGE:CXX>:/EHc->")

libobs/util/threading.h

@@ -26,12 +26,10 @@
 
 #include "c99defs.h"
 
-#ifdef _MSC_VER
-#include "../../deps/w32-pthreads/pthread.h"
-#else
+#ifndef _MSC_VER
 #include <errno.h>
-#include <pthread.h>
 #endif
+#include <pthread.h>
 
 #ifdef __cplusplus
 extern "C" {