Fix std::abs() usage

Call of overloaded 'abs(unsigned int&)' is ambiguous
See https://gcc.gnu.org/gcc-6/porting_to.html#overloaded-abs

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

libraries/CapacitiveSensor-0.5.1/CapacitiveSensor.cpp

@@ -61,7 +61,8 @@ long CapacitiveSensor::capacitiveSensor(uint8_t samples)
 		// only calibrate if time is greater than CS_AutocaL_Millis and total is less than 10% of baseline
 		// this is an attempt to keep from calibrating when the sensor is seeing a "touched" signal
 
-		if ( (millis() - lastCal > CS_AutocaL_Millis) && abs(total  - leastTotal) < (int)(.10 * (float)leastTotal) ) {
+		if ( (millis() - lastCal > CS_AutocaL_Millis) &&
+			 abs((signed long)total  - (signed long)leastTotal) < (int)(.10 * (float)leastTotal) ) {
 
 			// Serial.println();               // debugging
 			// Serial.println("auto-calibrate");