util/u_printf.h: fix build
Fixes build error with uClibc and gcc-9.3.0:
In file included from ../src/util/u_printf.cpp:24: ../src/util/u_printf.h:43:41: error: ‘va_list’ has not been declared 43 | size_t u_printf_length(const char *fmt, va_list untouched_args);
CC: 21.3