Build System: Ignore copies in copy-many-files if src == dst

BUG: 117224272

Test: 1. Modify build/core/base_rules.mk
         Add the following line after line 617
	  $(eval my_compat_dist_$(suite) += out/a:out/a))
      2. touch out/a
      3. make -j BluetoothInstrumentationTests
      4. It should build pass without any dependency errors.

Change-Id: I40372eab5b74e042f605c3cb38e72942531ed69a