procfs.c: Move solib_mappings_callback up to avoid compiler warning.

procfs.c: Move solib_mappings_callback up to avoid compiler warning.

There are currently 2 issues with the placement of this routine:
  - It's defined after it is being used (causing an implicit declaration);
  - It looks like it's being defined all the time, whereas it is used
    only on mips-irix (AFAICT) - shouldn't have this triggered a warning
    on sparc-solaris, for instance???

In any case, this patch moves this function up, inside the right region,
just before the function where it is actually used.

gdb/ChangeLog:

        * procfs.c (solib_mappings_callback): Move function up to avoid
        a compiler warning.