gallium: Ensure prototypes are wrapped in extern "C".

Fixes MSVC build failure due to inconsistent _ReadWriteBarrier
prototype.

src/gallium/include/pipe/p_compiler.h

@@ -60,6 +60,11 @@
 #include <stdbool.h>
 
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 #if !defined(__HAIKU__) && !defined(__USE_MISC)
 typedef unsigned int       uint;
 typedef unsigned short     ushort;
@@ -243,4 +248,10 @@ void _ReadWriteBarrier(void);
 #define unlikely(x) !!(x)
 #endif
 
+
+#if defined(__cplusplus)
+}
+#endif
+
+
 #endif /* P_COMPILER_H */