sigProcLib: Specify standard namespace for isnan()

In commit a3dce85f
"sigProcLib: Use explicit NaN check in sinc table generation"

Use of std::isnan(double) was added without namespace specifier,
which may cause build issues depending on whether the C version
isnan() call is available. Add standard namespace to force C++
call usage and potential build issues.

Change-Id: I49328c43fdd690a4e6a2b2e949411aaf5674ead1

Transceiver52M/sigProcLib.cpp

@@ -982,7 +982,7 @@ static void generateSincTable()
   for (int i = 0; i < TABLESIZE; i++) {
     auto x = (double) i / TABLESIZE * 8 * M_PI;
     auto y = sin(x) / x;
-    sincTable[i] = isnan(y) ? 1.0 : y;
+    sincTable[i] = std::isnan(y) ? 1.0 : y;
   }
 }