Include compile-internal.h in gcc-c-plugin.h

gcc-c-plugin.h refers to some types defined in compile-internal.h.
This patch changes the former to include the latter.

gdb/ChangeLog
2019-01-22  Tom Tromey  <tom@tromey.com>

	* compile/gcc-c-plugin.h: Include compile-internal.h.

gdb/compile/gcc-c-plugin.h

@@ -17,6 +17,8 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#include "compile-internal.h"
+
 /* A class representing the C plug-in.  */
 
 class gcc_c_plugin