libiberty: fix build of gdb with clang

This use a upstream fix at
https://gcc.gnu.org/ml/gcc-patches/2008-07/msg00292.html

BUG=475970
TEST=CC=x86_64-cros-linux-gnu-clang CXX=x86_64-cros-linux-gnu-clang++
     emerge-falco gdb

Change-Id: Ia46631625ad600adfe314ad6e36c154a10cfad4a

include/libiberty.h

diff --git a/include/libiberty.h b/include/libiberty.h
index f2cf573..6aa1f20 100644
--- a/include/libiberty.h
+++ b/include/libiberty.h
@@ -613,8 +613,10 @@
 /* Like sprintf but provides a pointer to malloc'd storage, which must
    be freed by the caller.  */
 
+#ifndef asprintf
 extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;
 #endif
+#endif
 
 #if !HAVE_DECL_VASPRINTF
 /* Like vsprintf but provides a pointer to malloc'd storage, which