aac_afh: Fix clang warning.

Although gcc seems to have no problem with it, clang complains about
illegal characters in string literals of aac_afh.c:

	aac_afh.c:114:25: warning: illegal character encoding in string literal [-Winvalid-source-encoding]
			if (!atom_cmp(type1, "<A9>ART"))
					      ^~~~
	aac_afh.c:116:30: warning: illegal character encoding in string literal [-Winvalid-source-encoding]
			else if (!atom_cmp(type1, "<A9>alb"))
						   ^~~~
	aac_afh.c:118:30: warning: illegal character encoding in string literal [-Winvalid-source-encoding]
			else if (!atom_cmp(type1, "<A9>nam"))
						   ^~~~
	aac_afh.c:120:30: warning: illegal character encoding in string literal [-Winvalid-source-encoding]
			else if (!atom_cmp(type1, "<A9>cmt"))
						   ^~~~
	aac_afh.c:122:30: warning: illegal character encoding in string literal [-Winvalid-source-encoding]
			else if (!atom_cmp(type1, "<A9>day"))

This patch encodes the offending value as "\xa9" to make clang happy,