fix compilation error about missing braces

IEC_TIMESPEC has two fields tv_sec and tv_nsec.

Compilation error:
./build/POUS.c:600:45: error: missing braces around initializer [-Werror=missing-braces]
static const STEP temp_step = {{0, 0}, 0, {0, 0}};
^
compilation terminated due to -Wfatal-errors.

include ../common.mk

lib_LIBRARIES = libabsyntax.a

libabsyntax_a_SOURCES = \
	absyntax.cc \
	visitor.cc