Added missing include to satisfy certain versions of clang

Some compilers, such as PS4's clang, were giving errors such as:
C:\dev\protobuf\src\google/protobuf/parse_context.h(227,5): error : no type named 'uintptr_t' in namespace 'std'; did you mean simply 'uintptr_t'?
Adding the necessary include for integer types fixes these.

src/google/protobuf/parse_context.h

@@ -33,6 +33,7 @@
 
 #include <cstring>
 #include <string>
+#include <cstdint>
 
 #include <google/protobuf/io/coded_stream.h>
 #include <google/protobuf/io/zero_copy_stream.h>