gdb.cp/call-c.exp C++ify

Make the testcase work when built with a C++ compiler.

gdb/testsuite/ChangeLog:

	* gdb.cp/call-c-1.c (foo) [__cplusplus]: Add extern "C".

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-09-17  Pedro Alves  <pedro@palves.net>
+
+	* gdb.cp/call-c-1.c (foo) [__cplusplus]: Add extern "C".
+
 2020-09-17  Pedro Alves  <pedro@palves.net>
 
 	* gdb.python/py-frame-inline.exp: Adjust to optionally expect a

gdb/testsuite/gdb.cp/call-c-1.c

@@ -15,4 +15,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#ifdef __cplusplus
+extern "C"
+#endif
 int foo(int x) { return x; }