lib: include strutils.h for mempcpy()

lib: include strutils.h for mempcpy()

We have that mempcpy fallback since 2013 (02887b73) but forgot to
include it.

This fixes a segfault of cal(1) on FreeBSD and OSX.

Compiler warning was:
lib/mbsalign.c:468:14: warning: implicit declaration of function 'mempcpy' is invalid in C99 [-Wimplicit-function-declaration]
      dest = mempcpy (dest, str_to_print, min (n_used_bytes, space_left));
             ^
lib/mbsalign.c:468:12: warning: incompatible integer to pointer conversion assigning to 'char *' from 'int' [-Wint-conversion]
      dest = mempcpy (dest, str_to_print, min (n_used_bytes, space_left));

CC: Daniel Trebbien <dtrebbien@gmail.com>
Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>