codecs/ilbc: Compile in Solaris 11.

The symbol FS is the sampling frequency. That symbol is not used in Asterisk at
all and was a copy-and-paste of the iLBC reference code from the IETF RFC.
However, in Solaris, that symbol is defined by another header already. To
compile in Solaris, that symbol has to go.

Change-Id: I91ddbe5be7c00069c3a25abd5f58d7b2f04c51b1