scanner: set errno=0 before strtol

POSIX says to set errno=0 before calling strtol since
the return value alne cannot tell a failure.

on ubuntu armel I get:

  ../src/wayland-scanner client-header < ../../protocol/wayland.xml > wayland-client-protocol.h
  <stdin>:1188: error: invalid integer (2)

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>