x86: pngdsp: Fix assembly for OS/2

The a.out object format does not allow aligning sections.
On OS/2 LD aligns sections to 16 bytes.
Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavcodec/x86/pngdsp.asm

@@ -28,7 +28,7 @@ SECTION_RODATA
 
 cextern pw_255
 
-section .text align=16
+SECTION_TEXT
 
 ; %1 = nr. of xmm registers used
 %macro ADD_BYTES_FN 1