Fixes the following error from GCC 7.2.0 with a recent newlib
In file included from /usr/arm-none-eabi/include/stdio.h:800:0,
from /home/jgn/work/src/riot/tests/unittests/tests-printf_float/tests-printf_float.c:21:
/home/jgn/work/src/riot/tests/unittests/tests-printf_float/tests-printf_float.c: In function ‘sfprintf_float’:
/home/jgn/work/src/riot/tests/unittests/tests-printf_float/tests-printf_float.c:39:28: error: ‘%f’ directive output truncated writing 11 bytes into a region of size 10 [-Werror=format-truncation=]
snprintf(str, BUFSIZE, "%f", in0);
^
/home/jgn/work/src/riot/tests/unittests/tests-printf_float/tests-printf_float.c:39:5: note: ‘__builtin_snprintf’ output 12 bytes into a destination of size 10
snprintf(str, BUFSIZE, "%f", in0);
^
cc1: all warnings being treated as errors
