lib/gcov-glue.c: Define macro `COVERAGE_MAGIC` and use it

The macro is defined in `util/cbmem/cbmem.c` too, so do the same here,
so that searching for that macro name shows all the usages.

Change-Id: I52e9fa414fbbe2012bc6d00312db528efba3e564
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/11803
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)
diff --git a/src/lib/gcov-glue.c b/src/lib/gcov-glue.c
index b193545..3ec7d7a 100644
--- a/src/lib/gcov-glue.c
+++ b/src/lib/gcov-glue.c
@@ -38,6 +38,8 @@
 
 #define COVERAGE_SIZE (32*1024)
 
+#define COVERAGE_MAGIC 0x584d41534
+
 static FILE *current_file = NULL;
 static FILE *previous_file = NULL;
 
@@ -56,7 +58,7 @@
 
 	// TODO check if we're at the end of the CBMEM region (ENOMEM)
 	if (current_file) {
-		current_file->magic = 0x584d4153;
+		current_file->magic = COVERAGE_MAGIC;
 		current_file->next = NULL;
 		if (previous_file)
 			previous_file->next = current_file;