Makefile: drop -Wno-universal-initializer from SP_EXTRA_FLAGS

Commit 1c96642326 ("sparse: allow '{ 0 }' to be used without warnings", 2020-05-22) added -Wno-universal-initializer to the SP_EXTRA_FLAGS in order to suppress potential sparse warnings from using '{0}' as an aggregate initializer. At that time, the default was for sparse to issue warnings (i.e. the default was -Wuniversal-initializer) if such an initializer was used to initialize an aggregate whose first member was a pointer type. However, this default was changed just a few days later to -Wno-universal-initializer (first released in sparse v0.6.2) and has been so in all subsequent release versions of sparse. Thus, including -Wno-universal-initializer in the SP_EXTRA_FLAGS variable is redundant. Remove the unnecessary warning flag from SP_EXTRA_FLAGS, essentially reverting commit 1c96642326. Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>