sqlite: fix AttributeError when +functions (#45327)

using self.compiler.cc_pic_flag here results in these errors:

==> sqlite: Executing phase: 'install'
==> Error: AttributeError: 'AutotoolsBuilder' object has no attribute 'compiler'

change it to self.pkg.compiler.cc_pic_flag instead.

var/spack/repos/builtin/packages/sqlite/package.py

@@ -279,7 +279,7 @@ def build_libsqlitefunctions(self):
             libraryname = "libsqlitefunctions." + dso_suffix
             cc = Executable(spack_cc)
             cc(
-                self.compiler.cc_pic_flag,
+                self.pkg.compiler.cc_pic_flag,
                 "-lm",
                 "-shared",
                 "extension-functions.c",