Added C standard `isprint()` function

sdk/include/ctype.h

diff --git a/sdk/include/ctype.h b/sdk/include/ctype.h
index 1011ad5..5861b28 100644
--- a/sdk/include/ctype.h
+++ b/sdk/include/ctype.h
@@ -3,6 +3,11 @@
 
 #pragma once
 
+static inline int isprint(int c)
+{
+	return c >= '\x20' && c <= '\x7e';
+}
+
 static inline int isdigit(int c)
 {
 	return c >= '0' && c <= '9';