rename elf.{c,h} into ltrace-elf.{c,h}

This patch renames these files to avoid a conflict with the header files
of the "libelfg0-dev" package. This avoids the "-iquote" magic.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
diff --git a/common.h b/common.h
index c672913..351ff80 100644
--- a/common.h
+++ b/common.h
@@ -7,7 +7,7 @@
 #include "dict.h"
 #include "sysdep.h"
 #include "debug.h"
-#include "elf.h"
+#include "ltrace-elf.h"
 #include "read_config_file.h"
 
 #if defined HAVE_LIBIBERTY || defined HAVE_LIBSUPC__