perf units: Fix insufficient array space

perf units: Fix insufficient array space

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: 9808143ba2e54818 ("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>