src-release.sh: Fix gdb source tarball build failure due to libsframe

src-release.sh: Fix gdb source tarball build failure due to libsframe

This script was recently changed as follow:

    | commit e619dddb3a45780ae66d762756882a3b896b617d
    | Date:   Tue Nov 15 15:07:13 2022 -0800
    | Subject: src-release.sh: Add libsframe
    |
    | Add libsframe to the list of top level directories that will be included
    | in a release.

Since then, the gdb source tarball has been failing with the error
below during the "make configure-host configure-target" phase:

    | make[3]: *** No rule to make target '../libsframe/libsframe.la',
    |     needed by 'libbfd.la'.  Stop.
    | make[3]: Leaving directory '/tmp/gdb-public/bfd'

This patch fixes the issue by adding libsframe to the list of
GDB_SUPPORT_DIRS, similar to what was done for BINUTILS.

ChangeLog:

        * src-release.sh (GDB_SUPPORT_DIRS): Add libsframe.