Fix linkage issue for ai_epsilon in C++20 modules (#6592)

* Add inline to ai_epsilon to avoid ODR violations
---------

Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>

include/assimp/defs.h

@@ -299,7 +299,7 @@ typedef unsigned int ai_uint;
 
 /* Numerical limits */
 #ifdef __cplusplus
-constexpr ai_real ai_epsilon = (ai_real) 1e-6;
+inline constexpr ai_real ai_epsilon = (ai_real) 1e-6;
 #else
 #  define ai_epsilon ((ai_real)1e-6)
 #endif