added GLM_FUNC_DECL to avoid nvcc warnings

when using glm with CUDA, nvcc prints out warning like these:

<pre>warning: a __host__ function("glm::detail::operator+") redeclared with __host__ __device__, hence treated as a __host__ __device__ function</pre>

This commit adds the missing <b>GLM_FUNC_DECL</b> to silence these warnings.