treewide: use full path to adc.h header

The build infrastructure should not be adding the drivers subdirectory
to the include path.  Fix the legacy uses that depended on that
addition.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>
diff --git a/drivers/adc/adc_shell.c b/drivers/adc/adc_shell.c
index 1431809..045e41d 100644
--- a/drivers/adc/adc_shell.c
+++ b/drivers/adc/adc_shell.c
@@ -6,7 +6,7 @@
 
 #include <shell/shell.h>
 #include <stdlib.h>
-#include <adc.h>
+#include <drivers/adc.h>
 #include <ctype.h>
 #include <sys/util.h>