ggml : add fallback definition for HWCAP2_SVE2 (#17683)

This align with other HWCAP2 feature flags

See #17528

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

ggml/src/ggml-cpu/arch/arm/cpu-feats.cpp

@@ -8,6 +8,10 @@
 #include <sys/sysctl.h>
 #endif
 
+#if !defined(HWCAP2_SVE2)
+#define HWCAP2_SVE2 (1 << 1)
+#endif
+
 #if !defined(HWCAP2_I8MM)
 #define HWCAP2_I8MM (1 << 13)
 #endif