x86inc: don't use read-only data sections on COFF targets

Yasm:
src/libavfilter/x86/af_volume.asm:24: warning: Standard COFF does not support read-only data sections
src/libavfilter/x86/af_volume.asm:24: warning: Unrecognized qualifier `align'

Nasm:
src/libavfilter/x86/af_volume.asm:24: error: standard COFF does not support section alignment specification
src/libavutil/x86/x86inc.asm:92: ... from macro `SECTION_RODATA' defined here

Tested-by: Clément Bœsch <u@pkh.me>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/x86/x86inc.asm

@@ -88,6 +88,8 @@
 %macro SECTION_RODATA 0-1 16
     %ifidn __OUTPUT_FORMAT__,aout
         section .text
+    %elifidn __OUTPUT_FORMAT__,coff
+        section .text
     %else
         SECTION .rodata align=%1
     %endif