gdbserver: configure + make clean fails (no 'common' subdir)

gdbserver: configure + make clean fails (no 'common' subdir)

This problem was noticed during the nightly snapshot build,
but can be reduced to:

        % /path/to/gdb/gdbserver/configure
        % make clean

The "make clean" fails, because it tries to do a "make clean"
in the common/ subdirectory.  But the problem is that this
common/ subdirectory is NOT created during the configure phase,
but rather during the build phase.  The error is fatal because
we had put the common subdirectory in the list of REQUIRED_SUBDIRS.

This fixes the problem by removing it from the REQUIRED_SUBDIRS
list.

gdb/gdbserver/ChangeLog:

        * Makefile.in (REQUIRED_SUBDIRS): Remove $(LIBCOMMON_DIR).