BLI: Use C++ guards for stack header

This is handy to have C++ guards for BLI functions so they
can be easily re-used in C++ code. This matches other headers
from this library as well.

source/blender/blenlib/BLI_stack.h

@@ -30,6 +30,10 @@
 
 #include "BLI_compiler_attrs.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef struct BLI_Stack BLI_Stack;
 
 BLI_Stack *BLI_stack_new_ex(
@@ -55,4 +59,8 @@ size_t BLI_stack_count(const BLI_Stack *stack) ATTR_WARN_UNUSED_RESULT ATTR_NONN
 
 bool BLI_stack_is_empty(const BLI_Stack *stack) ATTR_WARN_UNUSED_RESULT ATTR_NONNULL();
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif  /* __BLI_STACK_H__ */