Add an explicit cast to double when using fabs().

Commit bc43b7c2c0 used fabs() directly on an int variable, which
apparently requires an explicit cast on some platforms.

Per buildfarm.

src/backend/utils/adt/numeric.c

@@ -10290,7 +10290,7 @@ power_var_int(const NumericVar *base, int exp, NumericVar *result, int rscale)
 	 * to around log10(abs(exp)) digits, so work with this many extra digits
 	 * of precision (plus a few more for good measure).
 	 */
-	sig_digits += (int) log(fabs(exp)) + 8;
+	sig_digits += (int) log(fabs((double) exp)) + 8;
 
 	/*
 	 * Now we can proceed with the multiplications.