clover: Avoid using typename to allow compilation of clover by clang

clover: Avoid using typename to allow compilation of clover by clang

When parsing an variable declaration qualified with the typename
keyword, clang attempted to declare a variable with the type of non
type member "enum type type" of module::argument (within the header
file clover/core/module.hpp) instead of the typed member of
module::argument "enum type".

Replaced "typename" with "enum" to force clang to declare the variable
marg_type with type "enum type" of module::argument.

CC: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Albert Freeman <albertwdfreeman@gmail.com>