Include stdbool.h in headers which have functions with bool in signature

Done by manual inspecting of the output of this script:
grep -r -l -w "bool" * | grep 'c$' | sed 's/.c$//' > has_bool
grep -r -l -w "stdbool.h" * | grep 'h$' | sed 's/.h$//' > has_include
grep -F -x -v -f has_include has_bool

src/nvim/strings.h

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