gcc-plugins: Fix definition of DISABLE_LATENT_ENTROPY_PLUGIN

The variable DISABLE_LATENT_ENTROPY_PLUGIN is defined when
CONFIG_PAX_LATENT_ENTROPY is set. This is leftover from the original PaX
version of the plugin code and doesn't actually exist. Change the condition
to depend on CONFIG_GCC_PLUGIN_LATENT_ENTROPY instead.

Fixes: 38addce8 ("gcc-plugins: Add latent_entropy plugin")
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>