[sw, dif_rstmgr] Build DIF Rstmgr (was disabled)

Building of DIF Rstmgr was disabled by #3602, due to "the chicken and
egg problem". Both Reset Manager RTL and the DIF were merged, so
changing one of them would otherwise have caused a build failure. This
is why #3602 disabled the build temporarily, so that CI could pass.

Signed-off-by: Silvestrs Timofejevs <silvestrst@lowrisc.org>
diff --git a/sw/device/lib/dif/meson.build b/sw/device/lib/dif/meson.build
index 437112e..adf149c 100644
--- a/sw/device/lib/dif/meson.build
+++ b/sw/device/lib/dif/meson.build
@@ -129,19 +129,19 @@
 )
 
 # Reset Manager DIF library (dif_rstmgr)
-#sw_lib_dif_rstmgr = declare_dependency(
-#  link_with: static_library(
-#    'sw_lib_dif_rstmgr',
-#    sources: [
-#      hw_ip_rstmgr_reg_h,
-#      'dif_rstmgr.c'
-#    ],
-#    dependencies: [
-#      sw_lib_bitfield,
-#      sw_lib_mmio,
-#    ],
-#  )
-#)
+sw_lib_dif_rstmgr = declare_dependency(
+  link_with: static_library(
+    'sw_lib_dif_rstmgr',
+    sources: [
+      hw_ip_rstmgr_reg_h,
+      'dif_rstmgr.c'
+    ],
+    dependencies: [
+      sw_lib_bitfield,
+      sw_lib_mmio,
+    ],
+  )
+)
 
 # Alert handler library
 sw_lib_dif_alert_handler = declare_dependency(