AP_HAL: fixed build on g++ 7.3 for stm32

the system maths headers don't like our maths defines, but are happy
if we include math.h first

libraries/AP_HAL/AP_HAL_Macros.h

@@ -13,6 +13,9 @@
 #define ALLOW_DOUBLE_MATH_FUNCTIONS
 #endif
 
+// we need to include math.h here for newer compilers (eg. g++ 7.3.1 for stm32)
+#include <math.h>
+
 #if !defined(ALLOW_DOUBLE_MATH_FUNCTIONS)
 /* give warnings if we use double precision maths functions without
    specifying ALLOW_DOUBLE_TRIG_FUNCTIONS. Code should use the