xen: make clear gcov support limitation in Kconfig

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/Kconfig.debug

@@ -33,6 +33,12 @@ config GCOV
        ---help---
          Enable gcov (a test coverage program in GCC) support.
 
+         Currently the data structure and hypercall interface are tied
+         to GCC 3.4 gcov format. You need to have a version of GCC
+         that is compatible with that format to make gcov work.
+
+         If unsure, say N here.
+
 config LOCK_PROFILE
 	bool "Lock Profiling"
 	---help---