Fix build on MacOS-X 10.14: missing cmath header

This undoes a small part of commit 9ec0bcdcf2,
which seems a bit too agressive. Without the cmath header the build fails
using Apple clang version 11.0.0 (clang-1100.0.33.17) on Mojave with the
following error:

.../FreeCAD-git/src/Base/UnitsSchemaInternal.cpp:39:24: error: call to 'abs' is ambiguous
    double UnitValue = std::abs(quant.getValue());
                       ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/stdlib.h:132:6: note: candidate function
int      abs(int) __pure2;
         ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/stdlib.h:111:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long      abs(     long __x) _NOEXCEPT {return  labs(__x);}
                                           ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/stdlib.h:113:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {return llabs(__x);}
                                           ^
1 error generated.

src/Base/UnitsSchemaInternal.cpp

@@ -29,6 +29,7 @@
 #include <QString>
 
 #include "UnitsSchemaInternal.h"
+#include <cmath>
 
 
 using namespace Base;