Re-enable the OSI memory allocation tracker.

Previously, the OSI memory allocation tracker was disabled as
a temporary workaround for Bug 21561735.
The root cause for the original issue is now addressed by using the
appropriate linker flags when generating the bluetooth.default.so
shared library:

  LOCAL_LDLIBS := -Wl,-Bsymbolic,-Bsymbolic-functions

Also, added missing #include in file "btif/src/btif_sock_sco.c"

Bug: 21561735
Change-Id: I384a6f3d1d03c74f8d1e3c96bf7e0b5961206feb