Add missing headerh will be needed on Ubuntu

- closes https://github.com/assimp/assimp/issues/4720

include/assimp/Hash.h

@@ -47,6 +47,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #endif
 
 #include <stdint.h>
+#include <stdlib.h>
 #include <string.h>
 #include <cmath>
 
@@ -77,8 +78,8 @@ inline uint32_t SuperFastHash (const char * data, uint32_t len = 0, uint32_t has
     uint32_t tmp;
     int rem;
     
-    if (!data) return 0;
-    if (!len)len = (uint32_t)::strlen(data);
+    if (data == NULL) return 0;
+    if (len == 0)len = (uint32_t)::strlen(data);
 
     rem = len & 3;
     len >>= 2;