Fix VS2019 build (string module) (#2009)

Author: 1ndahous3

windows/vs2019/libyara/libyara.vcxproj

@@ -252,6 +252,7 @@
     <ClCompile Include="..\..\..\libyara\modules\pe\authenticode-parser\structs.c" />
     <ClCompile Include="..\..\..\libyara\modules\pe\authenticode-parser\certificate.c" />
     <ClCompile Include="..\..\..\libyara\modules\pe\pe_utils.c" />
+    <ClCompile Include="..\..\..\libyara\modules\string\string.c" />
     <ClCompile Include="..\..\..\libyara\modules\tests\tests.c" />
     <ClCompile Include="..\..\..\libyara\modules\time\time.c" />
     <ClCompile Include="..\..\..\libyara\notebook.c" />