Make prototypes static for autoclear unit test

With the explicit prototypes not marked as static unit test builds on Debian
Buster (using buster-backports) fail with

error: testing::internal::ParamGenerator<TestCase*> gtest_InstantiationAutoClearTestAutoClearTest_EvalGenerator_() was declared extern and later static [-fpermissive]

According to the comment preceding the declaration the only intent of the
explicit prototype (in newer versions of googletest implicitly defined by the
GTEST macros) seems to be to silence a warning which would cause a build
failure on Ubuntu 18.04 when using -Werror.

Builds on Debian unstable seem to build just fine without this explicit
declaration, hence I consider it safe to just drop it. However, builds on
Ubuntu 18.04 then fail (see above).

Making the prototypes as static should make both build environments happy.

libgnucash/app-utils/test/test-autoclear.cpp

@@ -157,9 +157,9 @@ TEST_P(AutoClearTest, DoesAutoClear) {
 }
 
 #ifndef INSTANTIATE_TEST_SUITE_P
-// Silence "no previous declaration for" which is treated as error, due to -Werror
-testing::internal::ParamGenerator<TestCase*> gtest_InstantiationAutoClearTestAutoClearTest_EvalGenerator_();
-std::string gtest_InstantiationAutoClearTestAutoClearTest_EvalGenerateName_(const testing::TestParamInfo<TestCase*>&);
+// Silence "no previous declaration for" (treated as error due to -Werror) when building with GoogleTest < 1.8.1
+static testing::internal::ParamGenerator<TestCase*> gtest_InstantiationAutoClearTestAutoClearTest_EvalGenerator_();
+static std::string gtest_InstantiationAutoClearTestAutoClearTest_EvalGenerateName_(const testing::TestParamInfo<TestCase*>&);
 
 INSTANTIATE_TEST_CASE_P(
 #else // INSTANTIATE_TEST_SUITE_P