arm64: neon: Export kernel_neon_busy to loadable modules

may_use_simd() can be invoked from loadable modules and it accesses
kernel_neon_busy. Make sure that the latter is exported.

Fixes: cb84d11e ("arm64: neon: Remove support for nested or hardirq kernel-mode NEON")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>