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

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