thermal: adc-tm: Fix compilation recipe in makefile

Correct compilation recipe in Makefile to pick all ADC_TM
driver files. Remove exported symbols as they are not needed
once compilation recipe is corrected.

Change-Id: I88ff9a13abf53460488683539fc925aa376fd6fa
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>