BaseTools/header.makefile: add "-Wno-stringop-truncation"

BaseTools/header.makefile: add "-Wno-stringop-truncation"

gcc-8 (which is part of Fedora 28) enables the new warning
"-Wstringop-truncation" in "-Wall". This warning is documented in detail
at <https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html>; the
introduction says

> Warn for calls to bounded string manipulation functions such as strncat,
> strncpy, and stpncpy that may either truncate the copied string or leave
> the destination unchanged.

It breaks the BaseTools build with:

> EfiUtilityMsgs.c: In function 'PrintMessage':
> EfiUtilityMsgs.c:484:9: error: 'strncat' output may be truncated copying
> between 0 and 511 bytes from a string of length 511
> [-Werror=stringop-truncation]
>          strncat (Line, Line2, MAX_LINE_LEN - strlen (Line) - 1);
>          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> EfiUtilityMsgs.c:469:9: error: 'strncat' output may be truncated copying
> between 0 and 511 bytes from a string of length 511
> [-Werror=stringop-truncation]
>          strncat (Line, Line2, MAX_LINE_LEN - strlen (Line) - 1);
>          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> EfiUtilityMsgs.c:511:5: error: 'strncat' output may be truncated copying
> between 0 and 511 bytes from a string of length 511
> [-Werror=stringop-truncation]
>      strncat (Line, Line2, MAX_LINE_LEN - strlen (Line) - 1);
>      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The right way to fix the warning would be to implement string concat with
snprintf(). However, Microsoft does not appear to support snprintf()
before VS2015
<https://stackoverflow.com/questions/2915672/snprintf-and-visual-studio-2010>,
so we just have to shut up the warning. The strncat() calls flagged above
are valid BTW.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Cole Robinson <crobinso@redhat.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>