Unresolved external symbol warning

I am trying to write a program for cypress FX2 chip. But I am getting the following warnings:

*** WARNING L1: UNRESOLVED EXTERNAL SYMBOL SYMBOL: USBIE MODULE: ex1.obj (EX1)
*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL SYMBOL: USBIE MODULE: ex1.obj (EX1) ADDRESS: 0807H

I have declared the header files containing the USBIE.
USBIE is defined in the header file as

EXTERN xdata volatile BYTE USBIE             _AT_ 0xE65C;  // USB Int Enables

What am I missing. The example files I have seen do not have any other include files.