This fixes build errors with external toolchains that don't
automatically search for headers in STAGING_DIR:

mipsel-linux-gnu-gcc   -I. -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
-D_GNU_SOURCE -DCOMP_DEFAULT=\"gzip\" -Wall -DGZIP_SUPPORT   -c -o
gzip_wrapper.o gzip_wrapper.c
gzip_wrapper.c:27:18: fatal error: zlib.h: No such file or directory
 #include <zlib.h>
                  ^
compilation terminated.
make[3]: *** [gzip_wrapper.o] Error 1

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>