Fix an issue with C-compatibility due to a missing tag name

system/core/libpixelflinger/include/pixelflinger/format.h:85:15: error: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Werror,-Wnon-c-typedef-for-linkage]
typedef struct {
              ^
              GGLFormat
system/core/libpixelflinger/include/pixelflinger/format.h:98:5: note: type is not C-compatible due to this member declaration
     inline uint32_t mask(int i) const {
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
system/core/libpixelflinger/include/pixelflinger/format.h:124:3: note: type is given name 'GGLFormat' for linkage purposes by this typedef declaration
} GGLFormat;
  ^

Bug: http://b/155835175
Test: m with aosp_blueline
Change-Id: I2819427e2c9e885f7b66527d5268987e8cc73f31
Merged-In: I73b95ff1d6d17af512ce2226b6324e5da0343c39