[gdb/testsuite] Fix regexp in gdb.base/foll-vfork.exp

On OBS I ran into:
...
(gdb) PASS: gdb.base/foll-vfork.exp: exit: \
  vfork relations in info inferiors: continue to child exit
info inferiors^M
  Num  Description       Connection           Executable        ^M
  1    <null>                                 foll-vfork-exit ^M
* 2    <null>                                 foll-vfork-exit ^M
(gdb) I'm the proud parent of child #5044!^M
FAIL: gdb.base/foll-vfork.exp: exit: vfork relations in info inferiors: \
  vfork relation no longer appears in info inferiors (timeout)
...

Fix this by removing the '$' anchor in the corresponding '$gdb_prompt $'
regexps.

Tested on x86_64-linux.