omap3evm: Wrap function under CONFIG_USB_OMAP3

The function omap3_evm_need_extvbus() is required
only when USB support is configured.

Wrapped this function in #ifdef CONFIG_USB_OMAP3.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>