crypto: ppc/curve25519 - add missing MODULE_DESCRIPTION() macro

Since commit 1fffe7a3 ("script: modpost: emit a warning when the
description is missing"), a module without a MODULE_DESCRIPTION() will
result in a warning with make W=1. The following warning is being
observed when building ppc64le with CRYPTO_CURVE25519_PPC64=m:

WARNING: modpost: missing MODULE_DESCRIPTION() in arch/powerpc/crypto/curve25519-ppc64le.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>