fix(turbo-quant): add forward declaration for turbo_cpu_fwht_inverse

CI failure (every -Werror build job — ubuntu-latest-cuda, ggml-ci-x64-cpu-*,
arm64-cpu-*, macOS-latest-{x64,arm64,arm64-webgpu}, ubuntu-22-{hip-quality-check,musa},
android-arm64, ubuntu-cpu x64-22.04):

  ggml/src/ggml-turbo-quant.c:247:15: error: no previous prototype for
  'turbo_cpu_fwht_inverse' [-Werror=missing-prototypes]

Pre-existing issue on the TQ branch — the function is defined GGML_API but
has no prototype declaration. M5 Max + M2 mini builds don't use -Werror so it
slid through local validation. Upstream CI does.

Fix: forward-declare the function near the top of the .c file. Matches the
extern declaration already used by tests/test-turbo-quant.c. No semantic change.

The other GGML_API symbol in this file (turbo3_cpu_wht_group_size) is a
variable, not a function — -Wmissing-prototypes does not apply.

Flagged by @pacak on PR TheTom#146 (ubuntu-latest-cuda CI). Restores green CI on
all -Werror build jobs without affecting any runtime path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-Authored-By: tturney@psyguard.ai

Author: 2 people

ggml/src/ggml-turbo-quant.c

@@ -20,6 +20,10 @@
 #define M_PI 3.14159265358979323846
 #endif
 
+/* Forward declarations for GGML_API symbols defined in this file (satisfies
+ * -Wmissing-prototypes under upstream CI's -Werror policy). */
+GGML_API void turbo_cpu_fwht_inverse(float * x, int group_size);
+
 /* Global: WHT group size for CPU quantize path (set by CPU SET_ROWS handler) */
 GGML_API int turbo3_cpu_wht_group_size = 0;