vulkan: request round-to-even for fp16 in im2col/rope_head (#10767)

Vulkan doesn't mandate a specific rounding mode, but the shader_float_controls
feature allows rounding mode to be requested if the implementation supports it.

ggml/src/ggml-vulkan/vulkan-shaders/rope_head.comp

@@ -1,6 +1,11 @@
 #include "types.comp"
 
 #extension GL_EXT_shader_16bit_storage : require
+#extension GL_EXT_spirv_intrinsics: enable
+
+#if RTE16
+spirv_execution_mode(capabilities = [4467], 4462, 16); // RoundingModeRTE, 16 bits
+#endif
 
 layout(local_size_x = 1, local_size_y = 256, local_size_z = 1) in;