Fix doc build dependencies for --with-system-readline

Fix doc build dependencies for --with-system-readline

PR build/30108 concerns building gdb documentation with
--with-sytem-readline.  If the in-tree readline directory is
missing, though, the docs will fail to build:

make[4]: Entering directory '/home/keiths/work/readline-doc-issue/linux/gdb/doc'
make[4]: *** No rule to make target '../../../src/gdb/doc/../../readline/readline/doc/rluser.texi', needed by 'gdb.info'.  Stop.

The listed file (and hsuser.texi) are conditionally included by gdb.texinfo.
When system readline is used, gdb/configure.ac will leave
READLINE_TEXI_INCFLAGS empty, causing doc/Makefile.in to output a line to
$BUILD/doc/GDBvn.texi with "@set SYSTEM_READLINE".  This surpresses the
inclusion of the missing files. They are not needed or used in this
scenario.

However, GDB_DOC_SOURCE_INCLUDES always lists these two files as dependencies,
thus provoking the build error whenever readline/ is missing.

This patch fixes this by creating (essentially) a conditional setting of the
dependencies to be included from readline.