[gdb/testsuite] Fix maint-expand-symbols-header-file.exp for cc-with-gdb-index

[gdb/testsuite] Fix maint-expand-symbols-header-file.exp for cc-with-gdb-index

With test-case gdb.base/maint-expand-symbols-header-file.exp and target board
cc-with-gdb-index, we have:
...
FAIL: gdb.base/maint-expand-symbols-header-file.exp: \
  verify no symtabs are expanded
...

By default, with partial symbols, we find the main function in the partial
symbols, and derive the initial language setting from that, without expanding
any psymtab.

But that doesn't work with the indices, because the indices don't store the
language with the symbols.  So instead, the main psymtab is expanded to get
the language of main, which causes the FAIL.

Fix this by manually setting the language.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-16  Tom de Vries  <tdevries@suse.de>

	* gdb.base/maint-expand-symbols-header-file.exp: Set language before
	loading exec.