tests: LMSDeviceTest: fix CPPFLAGS vs CXXFLAGS

* AM_CPPFLAGS is for preprocessor flags like '-I' or '-D',
* AM_CXXFLAGS is for C++ compiler flags like '-Wall'.

Change-Id: I82fc1eb6796f5289f0feff7bee5264bfacd6e733
diff --git a/tests/Transceiver52M/Makefile.am b/tests/Transceiver52M/Makefile.am
index a5b91f6..473cec6 100644
--- a/tests/Transceiver52M/Makefile.am
+++ b/tests/Transceiver52M/Makefile.am
@@ -33,5 +33,5 @@
 	$(COMMON_LA) \
 	$(LMS_LIBS) \
 	$(NULL)
-LMSDeviceTest_CPPFLAGS = $(AM_CPPFLAGS) $(LMS_CFLAGS)
+LMSDeviceTest_CXXFLAGS = $(AM_CFLAGS) $(LMS_CFLAGS)
 endif