When cross compiling with clang, the internal C++ headers are not found
when adding sysroot to -isystem, that is redundant anyway because it
will look for headers insider --sysroot path with same quality as it
would do with -isystem otherwise

Fixes errors like

FAILED: 3rdparty/openexr/CMakeFiles/IlmImf.dir/Iex/IexBaseExc.cpp.o
....
In file included from
TOPDIR/build/tmp/work/cortexa7t2hf-neon-vfpv4-bec-linux-musleabi/opencv/3.4.3+gitAUTOINC+b38c50b3_1f6d6f0626_bdb7bb85f3_34e4206aef_fccf7cd6a4-r0/git/3rdparty/openexr/Iex/IexBaseExc.cpp:43:
In file included from
TOPDIR/build/tmp/work/cortexa7t2hf-neon-vfpv4-bec-linux-musleabi/opencv/3.4.3+gitAUTOINC+b38c50b3_1f6d6f0626_bdb7bb85f3_34e4206aef_fccf7cd6a4-r0/git/3rdparty/openexr/Iex/IexBaseExc.h:48:
In file included from
TOPDIR/build/tmp/work/cortexa7t2hf-neon-vfpv4-bec-linux-musleabi/opencv/3.4.3+gitAUTOINC+b38c50b3_1f6d6f0626_bdb7bb85f3_34e4206aef_fccf7cd6a4-r0/recipe-sysroot/usr/lib//arm-bec-linux-musleabi/8.2.0/../../../include/c++/8.2.0/string:52:
In file included from
TOPDIR/build/tmp/work/cortexa7t2hf-neon-vfpv4-bec-linux-musleabi/opencv/3.4.3+gitAUTOINC+b38c50b3_1f6d6f0626_bdb7bb85f3_34e4206aef_fccf7cd6a4-r0/recipe-sysroot/usr/lib//arm-bec-linux-musleabi/8.2.0/../../../include/c++/8.2.0/bits/basic_string.h:6391:
In file included from
TOPDIR/build/tmp/work/cortexa7t2hf-neon-vfpv4-bec-linux-musleabi/opencv/3.4.3+gitAUTOINC+b38c50b3_1f6d6f0626_bdb7bb85f3_34e4206aef_fccf7cd6a4-r0/recipe-sysroot/usr/lib//arm-bec-linux-musleabi/8.2.0/../../../include/c++/8.2.0/ext/string_conversions.h:41:
TOPDIR/build/tmp/work/cortexa7t2hf-neon-vfpv4-bec-linux-musleabi/opencv/3.4.3+gitAUTOINC+b38c50b3_1f6d6f0626_bdb7bb85f3_34e4206aef_fccf7cd6a4-r0/recipe-sysroot/usr/lib//arm-bec-linux-musleabi/8.2.0/../../../include/c++/8.2.0/cstdlib:75:15:
fatal error: 'stdlib.h' file not found
              ^~~~~~~~~~
1 error generated.
Signed-off-by: Khem Raj <raj.khem@gmail.com>