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

examples/NonReg/Basics/STLTest/STLTest.ino

@@ -10,8 +10,8 @@
 */
 
 #include "test_stl.h"
-static uint32_t count_pressed = 0;
-static uint32_t random_val = 0;
+static int32_t count_pressed = 0;
+static int32_t random_val = 0;
 
 // Check if a default user button is defined
 // If not then define it to pin 2, user can set an other pin.