toolchain/buildroot: add comment for glibc NaN-2008 headers requirement

Commit 4a5140ecf (toolchain/buildroot: glibc requires kernel headers >=
4.5 with NaN-2008) added a restriction on kernel headers for glibc when
the architecture is using naN-2008.

However, such a restriction is usually associated to a comment explaining
the restriction, so the user knows what is happening.

That comment was forgotten in 4a5140ecf. Add it now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/toolchain-buildroot/Config.in

@@ -72,6 +72,11 @@ comment "glibc on powerpc64le needs a toolchain w/ headers >= 3.10"
 	depends on BR2_powerpc64le
 	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10
 
+# Support for MIPS NAN2008 needs headers >= 4.5
+comment "glibc on MIPS w/ NAN2008 needs a toolchain w/ headers >= 4.5"
+	depends on BR2_MIPS_NAN_2008
+	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_5
+
 config BR2_TOOLCHAIN_BUILDROOT_MUSL
 	bool "musl"
 	depends on BR2_aarch64	  || BR2_arm     || BR2_armeb     || BR2_i386 || \