[gdb/testsuite] Fix gdb.fortran/info-modules.exp with gcc-8

[gdb/testsuite] Fix gdb.fortran/info-modules.exp with gcc-8

When using test-case gdb.fortran/info-modules.exp with gcc 8.4.0, I run into:
...
FAIL: gdb.fortran/info-modules.exp: info module variables: check for entry \
  'info-types.f90', '35', 'Type m1t1 mod1::__def_init_mod1_M1t1;'
FAIL: gdb.fortran/info-modules.exp: info module variables: check for entry \
  'info-types.f90', '35', 'Type __vtype_mod1_M1t1 mod1::__vtab_mod1_M1t1;'
...

This is caused by this change in gdb output:
...
 (gdb) info module variables
    ...
 File gdb.fortran/info-types.f90:
-35:     Type m1t1 mod1::__def_init_mod1_M1t1;
+        Type m1t1 mod1::__def_init_mod1_M1t1;
-35:     Type __vtype_mod1_M1t1 mod1::__vtab_mod1_M1t1;
+        Type __vtype_mod1_M1t1 mod1::__vtab_mod1_M1t1;
 21:     real(kind=4) mod1::mod1_var_1;
 22:     integer(kind=4) mod1::mod1_var_2;
...
caused by a change in debug info.

Fix this by allowing those entries without line number.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-21  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/info-modules.exp (info module variables): Allow missing
	line numbers for some variables.