wgl: Wrap stw_pixelformat.h into extern C

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7535>

src/gallium/frontends/wgl/stw_pixelformat.h

@@ -38,6 +38,10 @@
 #include "pipe/p_format.h"
 #include "frontend/api.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct stw_pixelformat_info
 {
    PIXELFORMATDESCRIPTOR pfd;
@@ -68,4 +72,8 @@ stw_pixelformat_choose( HDC hdc,
 int
 stw_pixelformat_get(HDC hdc);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* STW_PIXELFORMAT_H */