vcxproj: require C11

This fixes the build after 7bc341e (git-compat-util: add a test
balloon for C99 support, 2021-12-01).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

contrib/buildsystems/Generators/Vcxproj.pm

@@ -178,6 +178,7 @@ sub createProject {
       <InlineFunctionExpansion>OnlyExplicitInline</InlineFunctionExpansion>
       <PrecompiledHeader />
       <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
+      <LanguageStandard_C>stdc11</LanguageStandard_C>
     </ClCompile>
     <Lib>
       <SuppressStartupBanner>true</SuppressStartupBanner>