fix(cross-ios): C{,XX}FLAGS: add missing $isysroot

I removed these in the previous refactor under the assumption that they
were not required, because $isysroot is also passed to CPPFLAGS.

Turns out I was wrong :^).

cross-ios

@@ -44,8 +44,8 @@ isysroot="-isysroot `xcrun -sdk $platform --show-sdk-path`"
 export CC="`xcrun -find -sdk $platform cc`"
 export CXX="`xcrun -find -sdk $platform g++`"
 export CPPFLAGS="-arch $architecture $isysroot $extra_cppflags $CPPFLAGS"
-export CFLAGS="-O2 -arch $architecture $minversion_flag $CFLAGS"
-export CXXFLAGS="-O2 -arch $architecture $minversion $CXXFLAGS"
+export CFLAGS="-O2 -arch $architecture $isysroot $minversion_flag $CFLAGS"
+export CXXFLAGS="-O2 -arch $architecture $isysroot $minversion $CXXFLAGS"
 export LDFLAGS="-arch $architecture $minversion $isysroot $LDFLAGS"
 
 $@ $configureflags