bindings.h: mark DEFAULT_BINDING_MODE as external

This prevents the following linker warning (only when compiling with
-fsanitize=address):

/usr/bin/ld: Warning: size of symbol `DEFAULT_BINDING_MODE' changed
from 8 in src/resize.o to 64 in src/bindings.o

include/bindings.h

@@ -15,7 +15,7 @@ extern pid_t command_error_nagbar_pid;
  * The name of the default mode.
  *
  */
-const char *DEFAULT_BINDING_MODE;
+extern const char *DEFAULT_BINDING_MODE;
 
 /**
  * Adds a binding from config parameters given as strings and returns a