Fix PHP 5.4 build

With PHP 5.4

    /builddir/build/BUILD/php-pecl-apcu-4.0.5/NTS/apc_bin.c: In function '_apc_swizzle_ptr':
    /builddir/build/BUILD/php-pecl-apcu-4.0.5/NTS/apc_bin.c:151:13: error: 'ptrdiff_t' undeclared (first use in this function)
             if((ptrdiff_t)bd < (ptrdiff_t)*ptr && (size_t)*ptr < ((size_t)bd + bd->size)) {

This seems related to missing include (don't really find the diff between 5.4 and 5.5+...)
As ptrdiff_t is known as not portable, the #else should fix the portability.