remove forward decl

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

Author: am17an

ggml/src/ggml-cuda/common.cuh

@@ -110,8 +110,6 @@
 #    define GGML_CUDA_USE_CUB
 #endif  // !defined(GGML_USE_HIP) && !defined(GGML_USE_MUSA) && CUDART_VERSION >= 11070
 
-size_t ggml_cuda_flash_attn_ext_get_alloc_size(int device, const ggml_tensor * dst);
-
 // PDL host-side support (cudaLaunchKernelEx) requires CUDART >= 11.8.
 // However, this has been bugged in CTK < 12.3 for MSVC builds, see
 // https://github.com/ggml-org/llama.cpp/pull/22522#discussion_r3302393293