libsrtp: do not include unnecessary sources

These two files should only be built when OpenSSL is not enabled. See
the configure script of the original sources and Makefile from upstream.
(RNG_EXTRA_OBJS)

Fixes a -Wimplicit-function-declaration warning.

libs/srtp/Makefile.am

@@ -15,7 +15,6 @@ libsrtp_la_SOURCES =    srtp/srtp.c srtp/ekt.c crypto/cipher/cipher.c crypto/cip
 			crypto/math/datatypes.c crypto/math/stat.c \
 			crypto/kernel/crypto_kernel.c crypto/kernel/alloc.c \
 			crypto/kernel/key.c \
-			crypto/rng/prng.c crypto/rng/ctr_prng.c \
 			crypto/kernel/err.c \
 			crypto/replay/rdb.c crypto/replay/rdbx.c crypto/replay/ut_sim.c 
 
@@ -30,6 +29,7 @@ libsrtp_la_SOURCES += crypto/hash/hmac_ossl.c
 else
 libsrtp_la_SOURCES += crypto/hash/sha1.c crypto/hash/hmac.c 
 libsrtp_la_SOURCES += crypto/cipher/aes_icm.c crypto/cipher/aes.c crypto/cipher/aes_cbc.c
+libsrtp_la_SOURCES += crypto/rng/prng.c crypto/rng/ctr_prng.c
 if RNG_OBJS_LINUX
 libsrtp_la_SOURCES += crypto/rng/rand_linux_kernel.c
 EXTRA_DIST += crypto/rng/rand_source.c