r300g: Fix build, invalid extern "C" around header inclusion.

A previous patch to fix header inclusion within extern "C" neglected
to fix the occurences of this pattern in r300 files.

When the helper to detect this issue was pushed to master, it broke
the build for the r300 driver.  This patch fixes the r300 build.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89477
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/drivers/r300/r300_public.h

@@ -2,8 +2,16 @@
 #ifndef R300_PUBLIC_H
 #define R300_PUBLIC_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct radeon_winsys;
 
 struct pipe_screen* r300_screen_create(struct radeon_winsys *rws);
 
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
 #endif