src: fix solaris 10 build error

Stop gcc from getting confused, explicitly cast the return value from
getuid() and getgid() to uint32_t.  Fixes the following build error:

    ../src/node.cc: In function 'void node::GetUid(const
    v8::FunctionCallbackInfo<v8::Value>&)':
    ../src/node.cc:1552:37: error: call of overloaded 'Set(uid_t)' is
    ambiguous
    ../src/node.cc:1552:37: note: candidates are:
    ../deps/v8/include/v8.h:5939:6: note: void
    v8::ReturnValue<T>::Set(bool) [with T = v8::Value]
    ../deps/v8/include/v8.h:5909:6: note: void
    v8::ReturnValue<T>::Set(double) [with T = v8::Value]
    ../deps/v8/include/v8.h:5915:6: note: void
    v8::ReturnValue<T>::Set(int32_t) [with T = v8::Value, int32_t = int]
    ../deps/v8/include/v8.h:5926:6: note: void
    v8::ReturnValue<T>::Set(uint32_t) [with T = v8::Value, uint32_t =
    unsigned int]

Fixes #6182.

src/node.cc

@@ -1549,12 +1549,14 @@ static gid_t gid_by_name(Handle<Value> value) {
 
 
 static void GetUid(const FunctionCallbackInfo<Value>& args) {
-  args.GetReturnValue().Set(getuid());
+  // uid_t is an uint32_t on all supported platforms.
+  args.GetReturnValue().Set(static_cast<uint32_t>(getuid()));
 }
 
 
 static void GetGid(const FunctionCallbackInfo<Value>& args) {
-  args.GetReturnValue().Set(getgid());
+  // gid_t is an uint32_t on all supported platforms.
+  args.GetReturnValue().Set(static_cast<uint32_t>(getgid()));
 }