Fix invalid extern "C" around header inclusion.

System headers may contain C++ declarations, which cannot be given C
linkage.  For this reason, include statements should never occur
inside extern "C".

This patch moves the C linkage statements to enclose only the
declarations within a single header.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/mapi/shared-glapi/tests/check_table.cpp

@@ -24,10 +24,8 @@
 #include <gtest/gtest.h>
 #include "../../../mesa/main/glheader.h"
 
-extern "C" {
 #include "glapi/glapi.h"
 #include "glapi/glapitable.h"
-}
 
 struct name_offset {
    const char *name;