[gd32/libraries]: fix compile error in rtc: "pmu_backup_write_enable" was not found. (#5906)

bsp/gd32/libraries/GD32F20x_Firmware_Library/SConscript

@@ -37,6 +37,7 @@ if GetDepend(['RT_USING_DAC']):
 
 if GetDepend(['RT_USING_RTC']):
     src += ['GD32F20x_standard_peripheral/Source/gd32f20x_rtc.c']
+    src += ['GD32F20x_standard_peripheral/Source/gd32f20x_pmu.c']
 
 if GetDepend(['RT_USING_WDT']):
     src += ['GD32F20x_standard_peripheral/Source/gd32f20x_wwdgt.c']

bsp/gd32/libraries/GD32F30x_Firmware_Library/SConscript

@@ -37,6 +37,7 @@ if GetDepend(['RT_USING_DAC']):
 
 if GetDepend(['RT_USING_RTC']):
     src += ['GD32F30x_standard_peripheral/Source/gd32f30x_rtc.c']
+    src += ['GD32F30x_standard_peripheral/Source/gd32f30x_pmu.c']
 
 if GetDepend(['RT_USING_WDT']):
     src += ['GD32F30x_standard_peripheral/Source/gd32f30x_wwdgt.c']

bsp/gd32/libraries/GD32F4xx_Firmware_Library/SConscript

@@ -38,6 +38,7 @@ if GetDepend(['RT_USING_DAC']):
 
 if GetDepend(['RT_USING_RTC']):
     src += ['GD32F4xx_standard_peripheral/Source/gd32f4xx_rtc.c']
+    src += ['GD32F4xx_standard_peripheral/Source/gd32f4xx_pmu.c']
 
 if GetDepend(['RT_USING_WDT']):
     src += ['GD32F4xx_standard_peripheral/Source/gd32f4xx_wwdgt.c']