android: make compat header self-contained

Change-Id: I0362725646521c9e2126faeaaf35a1c1dbf775af

include/android/compatibility.hxx

@@ -20,6 +20,9 @@
 #include <math.h>
 
 #if defined(ANDROID)
+#include <string>
+#include <sstream>
+
 namespace std
 {
 
@@ -65,4 +68,4 @@ template <typename T> std::string to_string(const T& rNumber)
     return aStream.str();
 }
 }
-#endif
\ No newline at end of file
+#endif