move 'use GPR2' statement to lsp-gpr_files.ads public part

workaround for eng/das/cov/gnatcoverage#174

(cherry picked from commit 92cf812)

Author: Philippe Gil

source/gpr/lsp-gpr_files.ads

@@ -47,6 +47,8 @@ with LSP.Tracers;
 
 package LSP.GPR_Files is
 
+   use GPR2; --  temporary workaround of a coverage instrumentation problem
+
    type File (Tracer : not null LSP.Tracers.Tracer_Access) is
      tagged limited private;
    --  A parsed GPR file.
@@ -215,8 +217,6 @@ package LSP.GPR_Files is
 
 private
 
-   use GPR2;
-
    type Source_Position is record
       Line   : Integer;
       Column : Integer;