ext/hls/meson.build: fix dependency logic

Currently, if one was to set -Dhls-crypto to either libgcrypt or openssl
instead of auto, the following lines would fail because hls_crypto_dep is not
yet set:

if not hls_crypto_dep.found() and ['auto', 'libgcrypt'].contains(hls_crypto)
if not hls_crypto_dep.found() and ['auto', 'openssl'].contains(hls_crypto)

Instead, change "if not hls_crypto_dep.found()" to "if not have_hls_crypto"
which fixes the error.