Add missing double quotes to fix build error on Windows

In this case, the value of "$%commonprogramfiles%" is treated
as 4 parameters (space separated), so FillPlaceHolder fails.

  !insertmacro: macro "FillPlaceHolder" requires 2 parameter(s), passed 5!
  Error in macro FillPlaceHolderWithTerms on macroline 1
  Error in script "fainstall.nsi" on line 2261 -- aborting creation process
  "Failed to build fainstall.exe!"