[Ada] Fix support for prefixed call with incomplete type declarations

[Pierre-Marie de Rodat]

GNAT crashes when using -gnatX on code with an incomplete type
declaration, due to a missing initialization of the list meant to store
primitive operations. Now fixed.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch3.adb (Analyze_Incomplete_Type_Decl): Add the missing
	initialization.
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