Avoid warning about missing math.h.

* Apparently caused by the fact that we no longer run fixincludes,
  math.h is not being generated anymore. I haven't removed the rm
  from the script in order to be compatible with older compilers.

build/scripts/build_cross_tools

@@ -161,7 +161,7 @@ make install-gcc-cross || {
 # Remove the math.h gcc header. It has been generated by fixincludes
 # (unconditional hack: math_huge_val_ifndef) from ours and it is semantically
 # equivalent.
-rm $installDir/lib/gcc-lib/i586-pc-haiku/$haikuRequiredLegacyGCCVersion/include/math.h
+rm -f $installDir/lib/gcc-lib/i586-pc-haiku/$haikuRequiredLegacyGCCVersion/include/math.h
 
 
 # cleanup