neatvnc: add nettle PACKAGECONFIG, disabled by default

The nettle crypto backend (RSA-AES security type) has no PACKAGECONFIG, so
meson auto-detects it from the sysroot: non-deterministic, nettle missing
from DEPENDS, and broken against nettle 4.0 (split sha.h, changed EAX
arity) which neatvnc 1.0.1 has not adopted.

Add the toggle with its nettle/hogweed/gmp deps and default it off, so the
build is deterministic and green on nettle 4.0; gnutls TLS is unaffected.
The real fix is a nettle-4.0 port of src/crypto/nettle in neatvnc upstream;
this only keeps the recipe building meanwhile.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-oe/recipes-graphics/neatvnc/neatvnc_1.0.1.bb

@@ -13,6 +13,7 @@ DEPENDS = "libdrm pixman aml zlib"
 
 PACKAGECONFIG ??= ""
 PACKAGECONFIG[tls] = "-Dtls=enabled,-Dtls=disabled,gnutls"
+PACKAGECONFIG[nettle] = "-Dnettle=enabled,-Dnettle=disabled,nettle hogweed gmp"
 PACKAGECONFIG[jpeg] = "-Djpeg=enabled,-Djpeg=disabled,libjpeg-turbo"
 PACKAGECONFIG[examples] = "-Dexamples=true,-Dexamples=false,libpng"
 PACKAGECONFIG[benchmarks] = "-Dbenchmarks=true,-Dbenchmarks=false,libpng"