meson.build: relax the libdl test to one for the function dlopen

For the *BSD family dlopen is already part of libc so it's not a hard
dependency to have a libdl.so library.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210714101536.16016-1-alex.bennee@linaro.org>