applied extern "C" guards to importerdesc.h

addresses #3685

include/assimp/importerdesc.h

@@ -52,6 +52,10 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <assimp/types.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** Mixed set of flags for #aiImporterDesc, indicating some features
   *  common to many importers*/
 enum aiImporterFlags {
@@ -143,4 +147,8 @@ Will return a nullptr if no assigned importer desc. was found for the given exte
 */
 ASSIMP_API const C_STRUCT aiImporterDesc *aiGetImporterDesc(const char *extension);
 
+#ifdef __cplusplus
+} // end of extern "C"
+#endif
+
 #endif // AI_IMPORTER_DESC_H_INC