libsamplerate: Define as PRIVATELIB

Fixes the following warnings:

ld: error: relocation R_X86_64_32S cannot be used against local symbol; recompile with -fPIC

defined in /usr/obj/usr/src/amd64.amd64/lib/libsamplerate/libsamplerate.a(samplerate.o)
referenced by samplerate.c:498 (/usr/src/contrib/libsamplerate/samplerate.c:498)

samplerate.o:(src_new) in archive /usr/obj/usr/src/amd64.amd64/lib/libsamplerate/libsamplerate.a

Related discussion: https://reviews.freebsd.org/D52306

Sponsored by: The FreeBSD Foundation
Reviewed by: fuz
Differential Revision: https://reviews.freebsd.org/D52307

(cherry picked from commit dafd960a3d65cdaccb1f8b9395fad14092ec9eb2)