ristretto: fix build error by including locale.h

build tested / error was:

| util.c: In function 'rstto_launch_help':
| util.c:36:35: error: 'LC_MESSAGES' undeclared (first use in this function)
| util.c:36:35: note: each undeclared identifier is reported only once for each function it appears in
| make[3]: *** [ristretto-util.o] Error 1
| make[3]: Leaving directory `/home/Superandy/tmp/oe-core-eglibc/work/armv7a-angstrom-linux-gnueabi/ristretto-0.3.0-r0/ristretto-0.3.0/src'
| make[2]: *** [all] Error 2
| make[2]: Leaving directory `/home/Superandy/tmp/oe-core-eglibc/work/armv7a-angstrom-linux-gnueabi/ristretto-0.3.0-r0/ristretto-0.3.0/src'
| make[1]: *** [all-recursive] Error 1
| make[1]: Leaving directory `/home/Superandy/tmp/oe-core-eglibc/work/armv7a-angstrom-linux-gnueabi/ristretto-0.3.0-r0/ristretto-0.3.0'
| make: *** [all] Error 2
| + die 'oe_runmake failed'

Signed-off-by: Andreas Müller <schnitzeltony@gmx.de>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>