Added -lpthread to target LDFLAGS for sim-eng.

This adds "-lpthread" to TARGET_GLOBAL_LDFLAGS when TARGET_SIMULATOR
is true.  This is much easier than inserting it into individual
makefiles as needed.  Has no effect on other builds.

Should fix the sim-eng build breakage in libcameraservice.

Change-Id: I4dcd7d54b93d1be1622b8ce78a1662d28ca9f9f2