powerpc: Fix missing declaration of [en/dis]able_kernel_altivec()

When CONFIG_PPC64 is set and CONFIG_ALTIVEC is not the following build
failures occur:

   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/dc_fpu.c: In function 'dc_fpu_begin':
>> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/dc_fpu.c:61:17: error: implicit declaration of function 'enable_kernel_altivec'; did you mean 'enable_kernel_vsx'? [-Werror=implicit-function-declaration]
      61 |                 enable_kernel_altivec();
         |                 ^~~~~~~~~~~~~~~~~~~~~
         |                 enable_kernel_vsx
   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/dc_fpu.c: In function 'dc_fpu_end':
>> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/dc_fpu.c:89:17: error: implicit declaration of function 'disable_kernel_altivec'; did you mean 'disable_kernel_vsx'? [-Werror=implicit-function-declaration]
      89 |                 disable_kernel_altivec();
         |                 ^~~~~~~~~~~~~~~~~~~~~~
         |                 disable_kernel_vsx
   cc1: some warnings being treated as errors

This commit adds stub instances of both enable_kernel_altivec() and
disable_kernel_altivec() the same way as done in commit bd737588
regarding enable_kernel_vsx() and disable_kernel_vsx().
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220221230741.293064-1-magalilemes00@gmail.com

arch/powerpc/include/asm/switch_to.h

@@ -62,6 +62,15 @@ static inline void disable_kernel_altivec(void)
 #else
 static inline void save_altivec(struct task_struct *t) { }
 static inline void __giveup_altivec(struct task_struct *t) { }
+static inline void enable_kernel_altivec(void)
+{
+	BUILD_BUG();
+}
+
+static inline void disable_kernel_altivec(void)
+{
+	BUILD_BUG();
+}
 #endif
 
 #ifdef CONFIG_VSX