pseudotalloc: Add simplistic implementation of talloc_asprintf

fsm.c uses this function. When compiled with --enable-embedded, it
triggers a warning:

fsm.c: In function 'osmo_fsm_inst_alloc':
fsm.c:213:4: warning: implicit declaration of function 'talloc_asprintf' [-Wimplicit-function-declaration]
    fi->name = talloc_asprintf(fi, "%s(%s)", fsm->name, id);

Change-Id: I61cd83d4c8570af1e452c945ac6f194baf7e6ac4
diff --git a/src/pseudotalloc/talloc.h b/src/pseudotalloc/talloc.h
index c7127de..f4c0a74 100644
--- a/src/pseudotalloc/talloc.h
+++ b/src/pseudotalloc/talloc.h
@@ -39,4 +39,4 @@
 			 size_t el_size,
 			 unsigned count,
 			 const char *name);
-
+char *talloc_asprintf(const void *ctx, const char *fmt, ...);