package/cryptopp: disable altivec if needed

Fix the following build failure raised since commit
bf4cf9de833e2d571d98d1ca935ea08a3468e463:

blake2s_simd.cpp:715:8: error: 'uint32x4_p' does not name a type; did you mean 'uint32_t'?
  715 | inline uint32x4_p VecLoad32(const T* p)
      |        ^~~~~~~~~~
      |        uint32_t

Fixes:
 - http://autobuild.buildroot.org/results/9e9f8559b12b8e2cf99cfab435fa7dd5796bbcd5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/cryptopp/cryptopp.mk

@@ -47,6 +47,10 @@ ifneq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_4_9),y)
 CRYPTOPP_CXXFLAGS += -DCRYPTOPP_DISABLE_AVX2
 endif
 
+ifneq ($(BR2_POWERPC_CPU_HAS_ALTIVEC),y)
+CRYPTOPP_CXXFLAGS += -DCRYPTOPP_DISABLE_ALTIVEC
+endif
+
 ifneq ($(BR2_ARM_CPU_HAS_NEON),y)
 CRYPTOPP_CXXFLAGS += -DCRYPTOPP_DISABLE_ARM_NEON
 endif