opkg: fix stray \ warnings with grep-3.8

opkg: fix stray \ warnings with grep-3.8

We simply grep for "src/". So no need for "\/".
Furthermore, since grep-3.8 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 a warning during the first boot:
  grep: warning: stray \ before /

Signed-off-by: Georgi Valkov <gvalkov@gmail.com>