Add include guard to filename-seen-cache.h

While moving things around, I stumbled on filename_seen_cache being
re-defined, because filename-seen-cache.h doesn't have an include guard.

gdb/ChangeLog:

	* filename-seen-cache.h: Add include guard.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2018-03-26  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* filename-seen-cache.h: Add include guard.
+
 2018-03-26  Keith Seitz  <keiths@redhat.com>
 
 	* symfile.c (place_section): Remove "struct" from section_addr_info

gdb/filename-seen-cache.h

@@ -17,6 +17,9 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#ifndef FILENAME_SEEN_CACHE_H
+#define FILENAME_SEEN_CACHE_H
+
 #include "defs.h"
 #include "common/function-view.h"
 
@@ -59,3 +62,5 @@ private:
   /* Table of files seen so far.  */
   htab_t m_tab;
 };
+
+#endif /* FILENAME_SEEN_CACHE_H */