mesa: Wrap extensions.h declarations with extern "C".

This should fix the MSVC linker failures that arose with commit
5e2d25894b.

Trivial.

src/mesa/main/extensions.h

@@ -38,6 +38,10 @@
 
 #include "glheader.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct gl_context;
 struct gl_extensions;
 
@@ -102,4 +106,8 @@ _mesa_has_##name_str(const struct gl_context *ctx) \
 extern struct gl_extensions _mesa_extension_override_enables;
 extern struct gl_extensions _mesa_extension_override_disables;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif