COMP: Suppress `-Wformat-nonliteral` warning in diff tensor example

[jhlegarreta]

Suppress -Wformat-nonliteral warning in diffusion tensor reconstruction image filter example.

Fixes:

[CTest: warning matched]
 /Users/builder/externalExamples/Filtering/DiffusionTensor3DReconstructionImageFilter.cxx:254:46:
 warning: format string is not a string literal [-Wformat-nonliteral]
        snprintf(filename, sizeof(filename), fn.c_str(), referenceImageIndex);
                                             ^~~~~~~~~~
[CTest: warning matched]
 /Users/builder/externalExamples/Filtering/DiffusionTensor3DReconstructionImageFilter.cxx:260:46:
 warning: format string is not a string literal [-Wformat-nonliteral]
        snprintf(filename, sizeof(filename), fn.c_str(), i);
                                             ^~~~~~~~~~
[CTest: warning suppressed] 2 warnings generated.

raised for example in: https://open.cdash.org/viewBuildError.php?type=1&onlydeltap&buildid=9582771

Left behind in commit 001a1a7.

PR Checklist

• [X] No API changes were made (or the changes have been approved)
• [X] No major design changes were made (or the changes have been approved)

[N-Dekker]

ITK.macos (https://open.cdash.org/viewBuildError.php?buildid=9584061) says:

Examples/Filtering/DiffusionTensor3DReconstructionImageFilter.cxx#L254:9: error: unknown type name 'ITK_GCC_PRAGMA_PUSH'

🤷 (But also please check my "inline" comments!)