lib/crypto: curve25519: Move a couple functions out-of-line
Move curve25519() and curve25519_generate_public() from curve25519.h to
curve25519.c. There's no good reason for them to be inline.
Link: https://lore.kernel.org/r/20250906213523.84915-10-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>