cgen: don't subtract null ptr in __offsetof, fix for v -cstrict -cc clang-14 cmd/tools/vpm.v and other programs using import net.http (fix #19222) (#19520)

Author: ttytm

vlib/v/gen/c/cheaders.v

@@ -360,7 +360,7 @@ const c_common_macros = '
 
 // for __offset_of
 #ifndef __offsetof
-	#define __offsetof(PTYPE,FIELDNAME) ((size_t)((char *)&((PTYPE *)0)->FIELDNAME - (char *)0))
+	#define __offsetof(PTYPE,FIELDNAME) ((size_t)(&((PTYPE *)0)->FIELDNAME))
 #endif
 
 #define OPTION_CAST(x) (x)