gdb: fix no-expat build of solib-target.c

Fixes:

      CXX    solib-target.o
    /home/smarchi/src/binutils-gdb/gdb/solib-target.c:57:8: error: ‘lm_info_vector’ does not name a type
       57 | static lm_info_vector
          |        ^~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/solib-target.c: In function ‘intrusive_list<shobj> solib_target_current_sos()’:
    /home/smarchi/src/binutils-gdb/gdb/solib-target.c:244:7: error: ‘solib_target_parse_libraries’ was not declared in this scope
      244 |     = solib_target_parse_libraries (library_document->data ());
          |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Ib477d3343b401017d79729118242143bc95f24b2

gdb/solib-target.c

diff --git a/gdb/solib-target.c b/gdb/solib-target.c
index e93fe8e..c612507 100644
--- a/gdb/solib-target.c
+++ b/gdb/solib-target.c
@@ -54,7 +54,7 @@ using lm_info_target_up = std::unique_ptr<lm_info_target>;
 
 #if !defined(HAVE_LIBEXPAT)
 
-static lm_info_vector
+static std::vector<lm_info_target_up>
 solib_target_parse_libraries (const char *library)
 {
   static int have_warned;
@@ -66,7 +66,7 @@ solib_target_parse_libraries (const char *library)
 		 "at compile time"));
     }
 
-  return lm_info_vector ();
+  return {};
 }
 
 #else /* HAVE_LIBEXPAT */