perf units: Fix insufficient array space

[ Upstream commit cf67629f7f637fb988228abdb3aae46d0c1748fe ]

No need to specify the array size, let the compiler figure that out.

This addresses this compiler warning that was noticed while build
testing on fedora rawhide:

  31    15.81 fedora:rawhide                : FAIL gcc version 15.0.1 20250225 (Red Hat 15.0.1-0) (GCC)
    util/units.c: In function 'unit_number__scnprintf':
    util/units.c:67:24: error: initializer-string for array of 'char' is too long [-Werror=unterminated-string-initialization]
       67 |         char unit[4] = "BKMG";
          |                        ^~~~~~
    cc1: all warnings being treated as errors

Fixes: 9808143b ("perf tools: Add unit_number__scnprintf function")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20250310194534.265487-3-acme@kernel.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>