#150 Fix build on macOS

[Maxim Khon]

Add missing #include <stddef.h> to fix the following error:</stddef.h>

gateway.c:845:18: error: call to undeclared function 'offsetof'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]

The build was done on macOS and privoxy was configured without openssl and brotli:

CPPFLAGS=-I/opt/local/include LDFLAGS=-L/opt/local/lib ./configure
The error does not happen when privoxy is configure with --with-openssl

[Fabian Keil]

Thanks a lot for the patch. Pushed to git master.