AP_ADC_AnalogSource: standardize inclusion of libaries headers

This commit changes the way libraries headers are included in source files:

 - If the header is in the same directory the source belongs to, so the
 notation '#include ""' is used with the path relative to the directory
 containing the source.

 - If the header is outside the directory containing the source, then we use
 the notation '#include <>' with the path relative to libraries folder.

Some of the advantages of such approach:

 - Only one search path for libraries headers.

 - OSs like Windows may have a better lookup time.

libraries/AP_ADC_AnalogSource/AP_ADC_AnalogSource.h

@@ -2,8 +2,8 @@
 #ifndef __AP_ADC_ANALOG_SOURCE_H__
 #define __AP_ADC_ANALOG_SOURCE_H__
 
-#include <AP_ADC.h>
-#include <AP_HAL.h>
+#include <AP_ADC/AP_ADC.h>
+#include <AP_HAL/AP_HAL.h>
 
 class AP_ADC_AnalogSource : public AP_HAL::AnalogSource
 {