vcpu_fp uses the riscv_isa_extension mechanism which gets
defined in hwcap.h but doesn't include that head file.

While it seems to work in most cases, in certain conditions
this can lead to build failures like

../arch/riscv/kvm/vcpu_fp.c: In function ‘kvm_riscv_vcpu_fp_reset’:
../arch/riscv/kvm/vcpu_fp.c:22:13: error: implicit declaration of function ‘riscv_isa_extension_available’ [-Werror=implicit-function-declaration]
   22 |         if (riscv_isa_extension_available(&isa, f) ||
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../arch/riscv/kvm/vcpu_fp.c:22:49: error: ‘f’ undeclared (first use in this function)
   22 |         if (riscv_isa_extension_available(&isa, f) ||

Fix this by simply including the necessary header.

Fixes: 0a86512d ("RISC-V: KVM: Factor-out FP virtualization into separate
sources")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Anup Patel <anup@brainfault.org>