Add missing newlines to "meson introspect" error

The lack of newlines in the print() statement for this error causes the result
to be jumbled and hard to read:

 $ ./meson.py introspect
 Current directory is not a meson build directory.Please specify a valid build dir or change the working directory to it.It is also possible that the build directory was generated with an oldmeson version. Please regenerate it in this case.

Add newlines so the message is more readable:

 $ ./meson.py introspect
 Current directory is not a meson build directory.
 Please specify a valid build dir or change the working directory to it.
 It is also possible that the build directory was generated with an old
 meson version. Please regenerate it in this case.

mesonbuild/mintro.py

@@ -398,9 +398,9 @@ def run(options):
 
     infofile = get_meson_info_file(infodir)
     if not os.path.isdir(datadir) or not os.path.isdir(infodir) or not os.path.isfile(infofile):
-        print('Current directory is not a meson build directory.'
-              'Please specify a valid build dir or change the working directory to it.'
-              'It is also possible that the build directory was generated with an old'
+        print('Current directory is not a meson build directory.\n'
+              'Please specify a valid build dir or change the working directory to it.\n'
+              'It is also possible that the build directory was generated with an old\n'
               'meson version. Please regenerate it in this case.')
         return 1