Fix comment for 'gdb_dlopen'

The 'gdb_dlopen' function doesn't return NULL if the shlib load
fails; it actually throws an error.  This patch updates the comment
to reflect this.

gdbsupport/ChangeLog:
2020-02-28  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb-dlfcn.h (gdb_dlopen): Update comment.

gdbsupport/ChangeLog

@@ -1,3 +1,7 @@
+2020-02-28  Sergio Durigan Junior  <sergiodj@redhat.com>
+
+	* gdb-dlfcn.h (gdb_dlopen): Update comment.
+
 2020-02-19  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* configure: Regenerate.

gdbsupport/gdb-dlfcn.h

@@ -32,8 +32,8 @@ struct dlclose_deleter
 typedef std::unique_ptr<void, dlclose_deleter> gdb_dlhandle_up;
 
 /* Load the dynamic library file named FILENAME, and return a handle
-   for that dynamic library.  Return NULL if the loading fails for any
+   for that dynamic library.  Throw an error if the loading fails for
-   reason.  */
+   any reason.  */
 
 gdb_dlhandle_up gdb_dlopen (const char *filename);