optimization: define GETNEXT as an inline macro

This reduces much function call overhead. getnext is defined in xml.h for
inline optimization. sfeed only uses one XML parser context per program, this
allows further optimizations of the compiler also.

On OpenBSD it was noticable because of retpoline etc function call overhead.
Using clang and a 500MB test XML file reduces processing time from +- 12s to
5s.

Tested using some crazy optimization flags:
SFEED_CFLAGS = -O3 -std=c99 -DGETNEXT=getchar_unlocked -fno-ret-protector \
	-mno-retpoline -static

A GETNEXT macro is also nice for programs which mmap(2) some big XML file. Then
you can simply define:

	#define GETNEXT() (off >= len ? EOF : reg[off++])

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