fix

pocketfft_hdronly.h

@@ -492,8 +492,8 @@ namespace threading {
 
 #ifdef POCKETFFT_NO_MULTITHREADING
 
-constexpr size_t thread_id() { return 0; }
-constexpr size_t num_threads() { return 1; }
+constexpr inline size_t thread_id() { return 0; }
+constexpr inline size_t num_threads() { return 1; }
 
 template <typename Func>
 void thread_map(size_t /* nthreads */, Func f)
@@ -501,12 +501,12 @@ void thread_map(size_t /* nthreads */, Func f)
 
 #else
 
-size_t &thread_id()
+inline size_t &thread_id()
   {
   static thread_local size_t thread_id_=0;
   return thread_id_;
   }
-size_t &num_threads()
+inline size_t &num_threads()
   {
   static thread_local size_t num_threads_=1;
   return num_threads_;