Fix unused variables
Warning on OSX reported by Peter Maydell on qemu mailing list:
../../subprojects/libslirp/src/slirp.c:131:17: error: unused variable 'old_stat' [-Werror,-Wunused-variable] struct stat old_stat; ^ ../../subprojects/libslirp/src/slirp.c:143:10: error: unused variable 'buff' [-Werror,-Wunused-variable] char buff[512]; ^ 2 errors generated.