build: ship stamp files

'make distcheck' fails because the generated ESX and HyperV files
are (intentionally) marked read-only, but since the stamp file was
missing, make assumes they need to be rebuilt.  Shipping the stamp
file solves the problem.

* src/Makefile.am (EXTRA_DIST): Ship stamp files.

src/Makefile.am

@@ -929,6 +929,8 @@ BUILT_SOURCES += $(ESX_DRIVER_GENERATED)
 
 ESX_GENERATED_STAMP = .esx_vi_generator.stamp
 
+EXTRA_DIST += $(ESX_GENERATED_STAMP)
+
 $(ESX_DRIVER_GENERATED): $(ESX_GENERATED_STAMP)
 
 $(ESX_GENERATED_STAMP): $(srcdir)/esx/esx_vi_generator.input \
@@ -955,6 +957,8 @@ BUILT_SOURCES += $(HYPERV_DRIVER_GENERATED)
 
 HYPERV_GENERATED_STAMP = .hyperv_wmi_generator.stamp
 
+EXTRA_DIST += $(HYPERV_GENERATED_STAMP)
+
 $(HYPERV_DRIVER_GENERATED): $(HYPERV_GENERATED_STAMP)
 
 $(HYPERV_GENERATED_STAMP): $(srcdir)/hyperv/hyperv_wmi_generator.input \