logrotate: needs mmu

Fix below build error:

logrotate.o: In function `switch_user_permanently':
/opt/test/buildroot/buildroot/output/build/logrotate-3.8.4/logrotate.c:129: undefined reference to `fork'
logrotate.o: In function `removeLogFile':
/opt/test/buildroot/buildroot/output/build/logrotate-3.8.4/logrotate.c:431: undefined reference to `fork'
logrotate.o: In function `mailLog':
/opt/test/buildroot/buildroot/output/build/logrotate-3.8.4/logrotate.c:597: undefined reference to `fork'
/opt/test/buildroot/buildroot/output/build/logrotate-3.8.4/logrotate.c:615: undefined reference to `fork'
logrotate.o: In function `compressLogFile':
/opt/test/buildroot/buildroot/output/build/logrotate-3.8.4/logrotate.c:533: undefined reference to `fork'
collect2: ld returned 1 exit status
make[1]: *** [logrotate] Error 1

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>