Fix declaration of extern int errno

Fix declaration of extern int errno

if HAVE_VSNPRINTF is declared, <errno.h> is not included.
So we need to declare it ourselves.

Error (Travis SCAN_BUILD=YES):
libtool: compile:  /usr/local/clang-7.0.0/bin/../libexec/ccc-analyzer -DHAVE_CONFIG_H -I. -I.. -g -O2 -Wextra -Wstrict-prototypes -Wmissing-prototypes -Wwrite-strings -Wno-variadic-macros -Wimport -Wfatal-errors -Wformat=2 -Winit-self -Wmissing-include-dirs -Wswitch-default -Wunknown-pragmas -MT snprintf.lo -MD -MP -MF .deps/snprintf.Tpo -c snprintf.c  -fPIC -DPIC -o .libs/snprintf.o
764snprintf.c:487:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
extern int errno;
^

Signed-off-by: Mikko Johannes Koivunalho <mikko.koivunalho@inoviagroup.se>