Fix internal compiler error with GCC 7

Here is the error message:
PX4/Firmware/src/systemcmds/tests/test_matrix.cpp:641:1: internal compiler error: in trunc_int_for_mode, at explow.c:55
 }
 ^
Please submit a full bug report,
with preprocessed source if appropriate.

src/systemcmds/tests/test_matrix.cpp

@@ -624,7 +624,9 @@ bool MatrixTest::vector2Tests()
 	ut_test(fabs(c(0) - 0) < 1e-5);
 	ut_test(fabs(c(1) - 0) < 1e-5);
 
-	Matrix<float, 2, 1> d(a);
+	static Matrix<float, 2, 1> d(a);
+	// the static keywork is a workaround for an internal bug of GCC
+	// "internal compiler error: in trunc_int_for_mode, at explow.c:55"
 	ut_test(fabs(d(0, 0) - 1) < 1e-5);
 	ut_test(fabs(d(1, 0) - 0) < 1e-5);