includes: prevent warnings (Z_CBPRINTF_ARG_SIZE shadows variables)

We compile with the gcc compiler flags -Werror and -Wshadow. So
the mentioned macro rasies a warning (and with the flags the
warning becoms to an error). It uses the same variable names
as on of its caller. Add an addidional underscore to the
affected variables to prevent this warning.

Signed-off-by: Peter Tönz <peter.tonz@husqvarnagroup.com>

include/zephyr/sys/cbprintf_internal.h

@@ -178,15 +178,15 @@ extern "C" {
 #define Z_CBPRINTF_ARG_SIZE(v) z_cbprintf_cxx_arg_size(v)
 #else
 #define Z_CBPRINTF_ARG_SIZE(v) ({\
-	__auto_type _v = (v) + 0; \
+	__auto_type __v = (v) + 0; \
 	/* Static code analysis may complain about unused variable. */ \
-	(void)_v; \
-	size_t _arg_size = _Generic((v), \
+	(void)__v; \
+	size_t __arg_size = _Generic((v), \
 		float : sizeof(double), \
 		default : \
-			sizeof((_v)) \
+			sizeof((__v)) \
 		); \
-	_arg_size; \
+	__arg_size; \
 })
 #endif