pass: add textflags.h if required

Closes #12

examples/add/add.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out add.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func Add(x uint64, y uint64) uint64
 TEXT ·Add(SB), $0-24
 	MOVQ	x(FP), AX

examples/args/args.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out args.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func StringLen(s string) int
 TEXT ·StringLen(SB), $0-24
 	MOVQ	s_len+8(FP), AX

examples/complex/complex.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out complex.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func Real(z complex128) float64
 TEXT ·Real(SB), $0-24
 	MOVSD	z_real(FP), X0

examples/dot/dot.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out dot.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func Dot(x []float32, y []float32) float32
 TEXT ·Dot(SB), $0-52
 	MOVQ	x_base(FP), AX

examples/fnv1a/fnv1a.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out fnv1a.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func Hash64(data []byte) uint64
 TEXT ·Hash64(SB), $0-32
 	MOVQ	data_base(FP), CX

examples/sha1/sha1.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out sha1.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func block(h *[5]uint32, m []byte)
 TEXT ·block(SB), $64-32
 	MOVQ	h(FP), AX

examples/stadtx/stadtx.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out stadtx.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func Hash(state *State, key []byte) uint64
 TEXT ·Hash(SB), $0-40
 	MOVQ	state(FP), AX

examples/sum/sum.s

@@ -1,7 +1,5 @@
 // Code generated by command: go run asm.go -out sum.s -stubs stub.go. DO NOT EDIT.
 
-#include "textflag.h"
-
 // func Sum(xs []uint64) uint64
 TEXT ·Sum(SB), $0-32
 	MOVQ	xs_base(FP), AX