Add missing include for integer types (#925)

Closes #924

`uintptr_t` and `uint32` (and possibly more) were undefined because
`cstdint` was not included.

Ast/include/Luau/Ast.h

@@ -8,6 +8,7 @@
 #include <string>
 
 #include <string.h>
+#include <stdint.h>
 
 namespace Luau
 {

Ast/src/StringUtils.cpp

@@ -7,6 +7,7 @@
 #include <vector>
 #include <string>
 #include <string.h>
+#include <stdint.h>
 
 namespace Luau
 {