[gdb/testsuite] Fix FAIL in gdb.base/list-missing-source.exp

In commit f8080fb7a4 "[gdb/testsuite] Add gdb.base/include-main.exp" a
file gdb.base/main.c was added, which caused the following regression:
...
(gdb) list^M
<gdb.base/main.c>
(gdb) FAIL: gdb.base/list-missing-source.exp: list
...

The problem is that the test-case does not expect to find a file main.c, but
now it finds gdb.base/main.c.

Fix this by using the more specific file name list-missing-source.c.

Tested on x86_64-linux.

gdb/testsuite/gdb.base/list-missing-source.exp

@@ -19,7 +19,7 @@
 standard_testfile
 
 # Create a source file in the output directory.
-set srcfile [standard_output_file main.c]
+set srcfile [standard_output_file list-missing-source.c]
 set fd [open "$srcfile" w]
 puts $fd {
 int