sal: fix Android build

Similar to 9a93277b (vcl: fix Android
build, 2015-03-06), where the Android has a broken C++ library, having
some functions in the global namespace instead of the std one.

Work around the problem by using the std namespace till it gets fixed on
the Android side.

Change-Id: I3d423f42b4e8e325beb3661c1780378e5edf962d

sal/rtl/math.cxx

@@ -38,6 +38,8 @@
 #include <stdlib.h>
 #include <cmath>
 
+using namespace std;
+
 static int const n10Count = 16;
 static double const n10s[2][n10Count] = {
     { 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8,