soc: qcom: qpnp-pbs: replace #ifdef with #if IS_ENABLED()

Replace #ifdef CONFIG_QPNP_PBS with #if IS_ENABLED(CONFIG_QPNP_PBS).
This fixes a function redefinition error when compiling the
qpnp-pbs driver as a module with CONFIG_QPNP_PBS=m in a defconfig
file.

Change-Id: I2a6201a7a94423ce333c203c475dce319f922fdd
Signed-off-by: David Collins <collinsd@codeaurora.org>