selftests/mlock2: add missing #define _GNU_SOURCE

On glibc 2.3.6:

    mlock2-tests.c: In function 'seek_to_smaps_entry':
    mlock2-tests.c:158: warning: implicit declaration of function 'getline'

According to the manpage of getline(), it needs _GNU_SOURCE before glibc
2.10.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Eric B Munson <emunson@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>