meson.build: fix libgcrypt detection on system without libgcrypt-config

meson.build: fix libgcrypt detection on system without libgcrypt-config

libgcrypt starts providing correct pkg-config configuration since 1.9,
in parallel with libgcrypt-config. Since 1.11 it may also stop
installing libgcrypt-config in some scenarios. Use the auto method for
detection of libgcrypt, in which meson will try both pkg-config and
libgcrypt-config.

Auto method for libgcrypt is supported by meson since 0.49.0, which is
higher than the version qemu requires.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>