pcre2: enable no MMU build

Use '--disable-pcre2grep-callout' for !BR2_USE_MMU, disables
fork usage.

Fixes [1]:

    CCLD     pcre2grep
  src/pcre2grep-pcre2grep.o: In function `pcre2grep_callout':
  pcre2grep.c:(.text+0x402): undefined reference to `fork'
  collect2: error: ld returned 1 exit status

[1] http://autobuild.buildroot.net/results/2c2/2c2665844748a3bdb010315200eea70aa3504b95



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>