Make private inlines in C files 'static inline', not just 'inline', or the compiler
can discard the definition if it chooses not to inline it.

Make functions declared in header files not be declared inline (if they're defined in a
.c file).  It looks like no functions in this category are used in LibSRTP's critical
path, only for unit tests or generating AES tables.

To see the problem prior to this commit, compile with "gcc -O0 -std=gnu99".
Signed-off-by: Travis Cross <tc@traviscross.com>

This cherry-picks commit e2774dbd551ffe5f872eaec2b2d40b712a54e1ba from
libsrtp upstream.

FS-6196 --resolve