Makefile.am: Fixed incorrect variable name if maintainer mode is active.

Signed-off-by: Slava Zanko <slavazanko@gmail.com>

src/Makefile.am

@@ -44,7 +44,7 @@ SRC_maintainer=logging.c logging.h
 SRC_charset = charsets.c charsets.h selcodepage.c selcodepage.h
 
 if USE_MAINTAINER_MODE
-    SRC_USE_maintainer=$(SRC_MAINTAINER)
+    SRC_USE_maintainer=$(SRC_maintainer)
 endif
 
 if CHARSET