core: Rename sounds_index.c to sounds.c.

This will make the source filename match the 'module reload sounds'
command.  This will allow conversion to a built-in module in Asterisk 16
without needing to redefine AST_MODULE.

Change-Id: Ifb8e489575b27eb33d8c0b6a531f266670557f6e

diff --git a/main/sounds_index.c b/main/sounds.c
similarity index 100%
rename from main/sounds_index.c
rename to main/sounds.c