Makefile: fix stray \ warnings with grep-3.8

Makefile: fix stray \ warnings with grep-3.8

We simply grep for "/usr". So no need for "-E" or "\/". Furthermore, in
the new grep versions this creates warnings.

As written in the grep-3.8 announcement:
  Regular expressions with stray backslashes now cause warnings, as
  their unspecified behavior can lead to unexpected results.
  For example, '\a' and 'a' are not always equivalent
  <https://bugs.gnu.org/39678>.

Fixes warnings in the form of:
  grep: warning: stray \ before /

Signed-off-by: Nick Hainke <vincent@systemli.org>