gdb: make symbols const in struct inline_state

Make the inline_state::skipped_symbols a vector of 'const symbol *',
adding the const qualifier.

There's only a couple of places this leaks into the rest of GDB and in
both places its fine for the symbol to become const.

There should be no functional change after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>