KVM: arm64: Fix symbol dependency in __hyp_call_panic_nvhe

__hyp_call_panic_nvhe contains inline assembly which did not declare
its dependency on the __hyp_panic_string symbol.

The static-declared string has previously been kept alive because of a use in
__hyp_call_panic_vhe. Fix this in preparation for separating the source files
between VHE and nVHE when the two users land in two different compilation
units. The static variable otherwise gets dropped when compiling the nVHE
source file, causing an undefined symbol linker error later.

Signed-off-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200625131420.71444-2-dbrazdil@google.com