Fix Vulkan SDK detection for MinGW cross-compilation

We must not set QMAKE_INCDIR_VULKAN if the Vulkan SDK is installed in
some default include directory. MinGW's std headers rely on
#include_next, which will break if we mess with the order of default
include paths.

Fixes: QTBUG-76660
Change-Id: I5ee0fc4c328ff88b979a8c1c010472b3883dff8d
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

mkspecs/features/win32/windows_vulkan_sdk.prf

@@ -3,6 +3,11 @@ isEmpty(QMAKE_INCDIR_VULKAN) {
     # headers are found out-of-the-box on typical Windows setups.
     QMAKE_INCDIR_VULKAN = $$(VULKAN_SDK)/include
 
+    # Do not add default include paths as that can knock std headers
+    # out of their stride due to their usage of #include_next.
+    contains(QMAKE_DEFAULT_INCDIRS, $$QMAKE_INCDIR_VULKAN): \
+        QMAKE_INCDIR_VULKAN =
+
     # Do not export the include dir but resolve it on every qmake call.
     QMAKE_EXPORT_INCDIR_VULKAN = -
 }