Include stdbool.h in some files which use it

Done by manual inspection of the output of this script:
grep -r -l -w "bool\|true\|false" * | grep 'c$\|h$' > has_bool
grep -r -l "stdbool.h" * | grep 'c$\|h$' > has_include
grep -F -x -v -f has_include has_bool

src/nvim/mbyte.h

@@ -1,6 +1,8 @@
 #ifndef NVIM_MBYTE_H
 #define NVIM_MBYTE_H
 
+#include <stdbool.h>
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "mbyte.h.generated.h"
 #endif