SockDiag - remove spurious class declaration

Fixes:
  system/netd/server/SockDiag.h:37:7:
  warning: no definition found for 'SockDiagTest',
  but a definition with the same name 'SockDiagTest'
  found in another namespace 'android::net'
  [bugprone-forward-declaration-namespace]

  class SockDiagTest;
        ^

Test: builds
Bug: 153035880
Change-Id: I1ff1b89a7066955457affe7f7cbbeef832ddc882
Merged-In: I1ff1b89a7066955457affe7f7cbbeef832ddc882
1 file changed