Avoid compiler warning using coding.h

* src/coding.h: Add INLINE_HEADER_BEGIN..INLINE_HEADER_END, since
this header now has an extern INLINE function.

src/coding.h

@@ -28,6 +28,8 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
 #include "lisp.h"
 
+INLINE_HEADER_BEGIN
+
 /* Index to arguments of Fdefine_coding_system_internal.  */
 
 enum define_coding_system_arg_index
@@ -768,4 +770,6 @@ extern struct coding_system safe_terminal_coding;
 
 extern char emacs_mule_bytes[256];
 
+INLINE_HEADER_END
+
 #endif /* EMACS_CODING_H */