Fix gdb.base/catch-fork-kill.c compilation

Fixes:

 Running .../src/gdb/testsuite/gdb.base/catch-fork-kill.exp ...
 gdb compile failed, .../src/gdb/testsuite/gdb.base/catch-fork-kill.c: In function 'main':
 .../src/gdb/testsuite/gdb.base/catch-fork-kill.c:81:4: warning: implicit declaration of function 'wait' [-Wimplicit-function-declaration]
     wait (NULL);
     ^

gdb/testsuite/ChangeLog:
2016-03-01  Pedro Alves  <palves@redhat.com>

	* gdb.base/catch-fork-kill.c: Include <sys/wait.h>.