faad2: fix build with musl libc

The getopt.c code declares the strncmp() routine in a non confirming way
under non GNU libc. Patch the code to make the declaration standard
conforming.

Fixes:
http://autobuild.buildroot.net/results/447/4471be349d7ad2e998a4d55afd33aa046a5d1fd2/
http://autobuild.buildroot.net/results/2a9/2a90f4f518884fb50f7ad6ab505dee7565ed869e/
http://autobuild.buildroot.net/results/6b1/6b159b766d791492bab4d897c33ce07845fb7119/



Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>