Add xtalloc_asprintf

I expect this to be useful in the upcoming implementation of token pasting.
diff --git a/glcpp.h b/glcpp.h
index 2e93cb9..048a9be 100644
--- a/glcpp.h
+++ b/glcpp.h
@@ -149,4 +149,7 @@
 char *
 xtalloc_strndup (const void *t, const char *p, size_t n);
 
+char *
+xtalloc_asprintf (const void *t, const char *fmt, ...);
+
 #endif