ggml-cpu: fix invalid hsum build in debug s390x (#15634)

Signed-off-by: Aaron Teo <aaron.teo1@ibm.com>

ggml/src/ggml-cpu/ggml-cpu-impl.h

@@ -489,7 +489,7 @@ inline static int16x8_t vec_padd_s16(int16x8_t a, int16x8_t b) {
 /**
  * @see https://github.com/ggml-org/llama.cpp/pull/14037
  */
-inline float vec_hsum(float32x4_t v) {
+inline static float vec_hsum(float32x4_t v) {
     float32x4_t v_temp = v + vec_reve(v);
     return v_temp[0] + v_temp[1];
 }