gdbsupport: use AC_FUNC_FORK instead of AC_FUNC_VFORK

gdbsupport: use AC_FUNC_FORK instead of AC_FUNC_VFORK

autoupdate does this change, it fixes this warning:

    configure.ac:50: warning: The macro `AC_FUNC_VFORK' is obsolete.
    configure.ac:50: You should run autoupdate.
    ../../lib/autoconf/functions.m4:1944: AC_FUNC_VFORK is expanded from...
    common.m4:20: GDB_AC_COMMON is expanded from...
    configure.ac:50: the top level

There are not changes in the generated configure files.

gdbsupport/ChangeLog:

	* common.m4: Replace AC_FUNC_VFORK with AC_FUNC_FORK.

Change-Id: I9de9f718c57e6d51c9734161f36c36ce39170325