[Plugin] Add C++ #ifdefs to public header files.

include/helper.h

@@ -27,7 +27,10 @@
 
 #ifndef ROFI_HELPER_H
 #define ROFI_HELPER_H
-
+#ifdef __cplusplus
+extern "C"
+{
+#endif
 #include <cairo.h>
 #include "rofi-types.h"
 
@@ -336,4 +339,7 @@ void parse_ranges ( char *input, rofi_range_pair **list, unsigned int *length );
  * @param filter
  */
 void rofi_output_formatted_line ( const char *format, const char *string, int selected_line, const char *filter );
+#ifdef __cplusplus
+}
+#endif
 #endif // ROFI_HELPER_H