Add guard to prevent eekboard/eekboard-*.h from being included directly.

eekboard/eekboard-context.h

@@ -15,6 +15,11 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
+
+#if !defined(__EEKBOARD_H_INSIDE__) && !defined(EEKBOARD_COMPILATION)
+#error "Only <eekboard/eekboard.h> can be included directly."
+#endif
+
 #ifndef EEKBOARD_CONTEXT_H
 #define EEKBOARD_CONTEXT_H 1