core: add "namespace cv" in CV_StaticAssert fallback implementation

modules/core/include/opencv2/core/cvdef.h

@@ -118,9 +118,11 @@ namespace cv { namespace debug_build_guard { } using namespace debug_build_guard
 #  if !defined(__clang__) && defined(__GNUC__) && (__GNUC__*100 + __GNUC_MINOR__ > 302)
 #    define CV_StaticAssert(condition, reason) ({ extern int __attribute__((error("CV_StaticAssert: " reason " " #condition))) CV_StaticAssert(); ((condition) ? 0 : CV_StaticAssert()); })
 #  else
+namespace cv {
      template <bool x> struct CV_StaticAssert_failed;
      template <> struct CV_StaticAssert_failed<true> { enum { val = 1 }; };
      template<int x> struct CV_StaticAssert_test {};
+}
 #    define CV_StaticAssert(condition, reason)\
        typedef cv::CV_StaticAssert_test< sizeof(cv::CV_StaticAssert_failed< static_cast<bool>(condition) >) > CVAUX_CONCAT(CV_StaticAssert_failed_at_, __LINE__)
 #  endif